y, _?, q, m, i

Key insight: each term decreases by 4 positions in the alphabet.

• y → 25. 25 − 4 = 21, which is u.

• u → 21. 21 − 4 = 17, which is q.

• q → 17. 17 − 4 = 13, which is m.

• m → 13. 13 − 4 = 9, which is i.

Therefore, the missing letter is u.