Select the letters from among the given options that can replace the question mark (?) in the following series.
K, M, O, ?, S, U, ?, Y

The correct option is Q, W.

To find the missing letters in the given letter series, let us analyze the pattern of the English alphabetical positions of each letter in the series.

Let us write down the alphabetical positions (where A = 1, B = 2, ..., Z = 26) for the given sequence:

K = 11

M = 13

O = 15

? (First missing letter)

S = 19

U = 21

? (Second missing letter)

Y = 25

By examining the differences between consecutive known terms:

$13-11=2$

$15-13=2$

$21-19=2$

$25-21=4$ (which accounts for two jumps of +2)

We can observe a clear pattern where each letter is obtained by adding 2 to the position of the previous letter (moving forward by 2 steps in the alphabet).

Step 1: Finding the first missing letter
Adding 2 to the position of O (15):

$15+2=17$

The 17^th letter of the English alphabet is Q.

Checking the transition from Q (17) to S (19): $17+2=19$, which matches S.

Step 2: Finding the second missing letter
Adding 2 to the position of U (21):

$21+2=23$

The 23^rd letter of the English alphabet is W.

Checking the transition from W (23) to Y (25): $23+2=25$, which matches Y.

Therefore, the missing letters that replace the question marks are Q and W respectively.