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

Correct — B.
Rule: +2 at every step (one letter skipped each time).

K(11), M(13), O(15), Q(17), S(19), U(21), W(23), Y(25).

Both blanks fit their neighbours: O + 2 = Q, Q + 2 = S, U + 2 = W, W + 2 = Y.

The blanks are Q, W → option (b).

Why the others are wrong

• (a)
  Q, X — Q, X gets the first blank right, but U(21) to X(24) is +3 and X to Y only +1. The second blank must be U + 2 = W.
• (c)
  P, W — P, W gets the second blank right, but O(15) to P(16) is only +1 and P to S is +3. The first blank must be O + 2 = Q.
• (d)
  R, W — R, W puts R(18) in the first blank: O to R is +3, then R to S only +1. The step is +2, so the blank is Q.

A single-letter series is a number series in disguise. Write each letter's position (A = 1 … Z = 26) and look at the gaps.

Here the known positions are 11, 13, 15, 19, 21 and 25, so every gap is 2. A blank with letters on both sides must fit both gaps: the first blank sits between O and S, the second between U and Y.

Key facts

• K = 11, M = 13, O = 15, Q = 17, S = 19, U = 21, W = 23, Y = 25.
• A step of +2 skips exactly one letter: K, (L), M, (N), O.
• A blank with known letters on both sides must fit the gap on each side.

Common traps

• Settling the first blank and taking the first option that has it, which here is Q, X
• Treating K to M as a step of 1 because only one letter is skipped

No directly related past PYQ was found.