Select the letter that can replace the question mark (?) in the following series.

W, U, R, ?, I, C

Series-  W, U, R, ?, I, C

Logic: The differences between consecutive letters are 2, 3, 4, 5, 6

W, U, R, ?, I, C 

From W (23) to U (21): 23−21=2

From U (21) to R (18): 21−18=3

From R (18) to ?: Difference increases by 1, so 18−4= 14 (N).

From N (14) to I (9): 14−9=5

From I (9) to C (3): 9−3=6

The missing letter is N.

Thus, correct answer is (a) N