Identifying the Letter Series Pattern

The question presents a letter series: J, S, B, K, ?. We need to find the pattern to determine the missing letter.

Analyzing the Alphabetical Positions

First, let's convert the letters to their corresponding positions in the English alphabet:

• J is the 10^th letter.
• S is the 19^th letter.
• B is the 2^nd letter.
• K is the 11^th letter.

Determining the Pattern Rule

Let's examine the difference between the positions of consecutive letters. We'll consider the alphabet as a cycle (A=1, ..., Z=26).

• From J (10) to S (19): The difference is $19 - 10 = +9$.
• From S (19) to B (2): Moving forward, $19 + 9 = 28$. Since there are 26 letters, we wrap around: $28 \pmod{26} = 2$. The 2^nd letter is B. The difference is $+9$.
• From B (2) to K (11): The difference is $11 - 2 = +9$.

The consistent pattern is adding 9 to the alphabetical position of the current letter to get the position of the next letter, with wrap-around.

Calculating the Missing Letter

To find the letter that replaces the question mark, we apply the pattern to the last known letter, K (11):

• Current letter: K (position 11)
• Apply the pattern: $11 + 9 = 20$.
• The 20^th letter of the alphabet is T.

Conclusion

Therefore, the letter T replaces the question mark to complete the series.