This question asks us to find the relationship between the first pair of letters (PQRS and QSUW) and apply that same relationship to the third term (ABCD) to find the fourth term.

Understanding the Letter Analogy Pattern

Let's look at the position of each letter in the English alphabet to find the pattern connecting PQRS to QSUW.

From the table, we can see a clear pattern. The shift in the position of the letters increases by one for each subsequent letter: the first letter shifts by $+1$, the second by $+2$, the third by $+3$, and the fourth by $+4$.

Applying the Pattern to ABCD

Now, we will apply this $+1, +2, +3, +4$ positional shift pattern to the letters ABCD.

Applying the pattern to ABCD gives us the letters BDFH.

Conclusion

The pattern observed between PQRS and QSUW is a positional shift of $+1, +2, +3, +4$ respectively. Applying this same pattern to ABCD results in BDFH. Therefore, the missing term in the analogy ABCD : ? is BDFH.

Revision Table: Key Concepts Reviewed

Additional Information: Types of Letter Series & Analogies

Letter series and analogy questions are common in logical reasoning tests. They can follow various patterns:

• Positional shifts (like in this question, adding/subtracting positions).
• Skipping letters (e.g., A, C, E, ... skipping one letter each time).
• Reverse order of letters.
• Vowel/consonant patterns.
• Combination of patterns.

To solve these, writing down the alphabet with its positions (1-26) is often helpful. Practice identifying different types of patterns.