Q1F, S2E, U6D, W21C, ?

Each term has three parts: a first letter, a number, and a last letter. Track the three series separately.

First letter: Q → S → U → W. These are at positions 17, 19, 21, 23, increasing by +2 each time, so the next is 25 = Y.

Number: 1, 2, 6, 21, ? Each number is obtained from the previous one by the rule aₙ = aₙ₋₁ × k + k, where k = 1, 2, 3, 4. So 1×1+1 = 2, 2×2+2 = 6, 6×3+3 = 21, and 21×4+4 = 88.

Last letter: F → E → D → C. These are at positions 6, 5, 4, 3, decreasing by −1 each time, so the next is 2 = B.

Combining the three parts gives the missing term: Y88B.