To complete the given series, we need to find the combination of letters that fits into the blanks following a specific pattern.

Analyzing the Given Letter Series

The given series with blanks is:

r _ k n _c _ _ h _ _ b c b t _ k _ b c b

There are 8 blanks that need to be filled. We are given four options, each providing a sequence of 8 letters.

Testing the Correct Option

Let's test the letters from the correct option, which is: h, b, b, s, k, n, h, n.

We place these letters into the blanks sequentially:

• The 1st blank (position 2) is filled with h.
• The 2nd blank (position 5) is filled with b.
• The 3rd blank (position 7) is filled with b.
• The 4th blank (position 8) is filled with s.
• The 5th blank (position 10) is filled with k.
• The 6th blank (position 11) is filled with n.
• The 7th blank (position 16) is filled with h.
• The 8th blank (position 18) is filled with n.

Filling these letters into the blanks of the original series gives the complete sequence:

r h k n b c b s h k n b c b t h k n b c b

Identifying the Pattern in the Completed Series

Let's examine the completed series r h k n b c b s h k n b c b t h k n b c b to identify the underlying pattern.

Upon observation, the series can be broken down into segments:

Segment 1: r h k n b c b s (Length 8)

Segment 2: h k n b c b t (Length 7)

Segment 3: h k n b c b (Length 6)

Remaining part from original series: b c b (Length 3, positions 22-24)

Concatenating these parts gives the full series: r h k n b c b s h k n b c b t h k n b c b b c b which is too long (8+7+6+3=24, so the last `bcb` must be part of Segment 3 or the overall structure). Let's re-verify the positions in the filled string:

r h k n b c b s h k n b c b t h k n b c b (Total 24 characters)

Let's look at the core repeating block h k n b c b. It appears multiple times.

The pattern is formed by concatenating the following parts:

• The letter r (Position 1)
• The sequence h k n b c b (Positions 2-7)
• The letter s (Position 8)
• The sequence h k n b c b (Positions 9-14)
• The letter t (Position 15)
• The sequence h k n b c b (Positions 16-21)
• The sequence b c b (Positions 22-24, which are the final letters of the original series)

Let's combine these parts sequentially:

r + hknbcb + s + hknbcb + t + hknbcb + bcb

This concatenation results in: r h k n b c b s h k n b c b t h k n b c b b c b. This is 27 characters long, which is incorrect.

Let's rely on the confirmed filled series and describe its structure accurately:

The series is r h k n b c b s h k n b c b t h k n b c b.

This sequence contains the block h k n b c b repeated three times.

• The first instance of h k n b c b is preceded by r and followed by s.
• The second instance of h k n b c b is preceded by the h that immediately follows s, and is followed by t.
• The third instance of h k n b c b is preceded by the h that immediately follows t, and concludes the patterned part of the series. The fixed characters b c b from the original series complete the sequence.

The structure can be visually represented as:

(r) (h k n b c b) (s) (h k n b c b) (t) (h k n b c b)

Checking the lengths: 1 + 6 + 1 + 6 + 1 + 6 = 21 characters. The total length is 24. The final 3 characters b c b must be considered. They are part of the original series structure at the end.

So, the correct pattern structure is the concatenation of:

• r
• h k n b c b
• s
• h k n b c b
• t
• h k n b c b
• The final fixed b c b from the original sequence template.

Let's apply this pattern template to the blanks:

Original: r _ k n _c _ _ h _ _ b c b t _ k _ b c b

Structure: r + (h k n b c b) + s + (h k n b c b) + t + (h k n b c b) + bcb

Comparing this structure with the original series template allows us to determine the letters in the blanks:

r matches r (pos 1)

h k n b c b should fill _ k n _c _ _ (pos 2-8 minus pos 3,4,6 which are k,n,c)

• Pos 2: blank -> should be h
• Pos 3-4: kn -> matches kn
• Pos 5: blank -> should be b
• Pos 6: c -> matches c
• Pos 7: blank -> should be b
• Pos 8: blank -> should be s (This matches the 's' after the first 'hknbcb' block in the pattern)

Next part: h k n b c b should fill h _ _ b c b (pos 9-14)

• Pos 9: h -> matches h (This 'h' is the start of the second 'hknbcb' block in the pattern)
• Pos 10: blank -> should be k
• Pos 11: blank -> should be n
• Pos 12-14: bcb -> matches bcb

Next part: t (pos 15)

• Pos 15: t -> matches t

Next part: h k n b c b should fill _ k _ b c b (pos 16-21 minus pos 17, 19-21 which are k, bcb)

• Pos 16: blank -> should be h (This 'h' is the start of the third 'hknbcb' block in the pattern)
• Pos 17: k -> matches k
• Pos 18: blank -> should be n
• Pos 19-21: bcb -> matches bcb

Final part: b c b (pos 22-24)

• Pos 22-24: bcb -> matches bcb

The letters filling the blanks are from the structure derived: h, b, b, s, k, n, h, n.

The sequence of letters filling the blanks is h b b s k n h n, which matches the tested option.

Conclusion

The combination of letters h b b s k n h n correctly completes the series r _ k n _c _ _ h _ _ b c b t _ k _ b c b by forming the pattern based on the repeating sequence h k n b c b with specific surrounding letters.

Revision Table: Letter Series Patterns

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Additional Information: Strategies for Series Questions

• Count the total number of elements and the number of blanks. This can sometimes suggest the length of repeating blocks if the series length is a multiple of a small number.
• Write down the alphabet (A-Z) to easily count skips or identify sequences.
• Look for common sequences like 'abc', 'def', 'pqr', etc., or reversals like 'zyx'.
• If options are provided, try substituting them into the blanks one by one and then look for a pattern in the resulting complete series. This is often the most reliable method for complex series.
• Break down the series into smaller parts, especially if certain letters appear at regular intervals.