Select the letter that can replace the question mark (?) in the following series.
F, I, K, N, P, S, U, ?

Options

A. Q
B. P
C. S
D. X

Answer

To solve the problem, we need to identify the pattern in the given series of letters: F, I, K, N, P, S, U, ?.

### Step-by-Step Solution:

1. **Identify the Series**: The series is F, I, K, N, P, S, U.

2. **Determine the Position of Each Letter**:
- F is the 6th letter of the alphabet.
- I is the 9th letter of the alphabet.
- K is the 11th letter of the alphabet.
- N is the 14th letter of the alphabet.
- P is the 16th letter of the alphabet.
- S is the 19th letter of the alphabet.
- U is the 21st letter of the alphabet.

3. **Calculate the Differences**:
- From F (6) to I (9): +3
- From I (9) to K (11): +2
- From K (11) to N (14): +3
- From N (14) to P (16): +2
- From P (16) to S (19): +3
- From S (19) to U (21): +2

4. **Identify the Pattern**: The pattern alternates between adding 3 and adding 2:
- +3, +2, +3, +2, +3, +2

5. **Apply the Pattern to U**:
- Since the last operation was +2 (from S to U), the next operation should be +3.
- U (21) + 3 = 24.

6. **Find the Corresponding Letter**:
- The 24th letter of the alphabet is X.

Thus, the letter that can replace the question mark (?) in the series is **X**.

### Final Answer: X