Which letter cluster will replace the question mark (?) to complete the given series?
HEWC, KIRI, ?, QQHU, TUCA

The correct option is NMMO.

To determine the letter cluster that replaces the question mark (?) in the series HEWC, KIRI, ?, QQHU, TUCA, we can analyze the positional pattern of each letter in the clusters based on their alphabetical positions (where A = 1, B = 2, ..., Z = 26):

1st letter of each cluster:
H (8) + 3 = K (11)
K (11) + 3 = N (14)
N (14) + 3 = Q (17)
Q (17) + 3 = T (20)
Thus, the 1st letter is N.

2nd letter of each cluster:
E (5) + 4 = I (9)
I (9) + 4 = M (13)
M (13) + 4 = Q (17)
Q (17) + 4 = U (21)
Thus, the 2nd letter is M.

3rd letter of each cluster:
W (23) - 5 = R (18)
R (18) - 5 = M (13)
M (13) - 5 = H (8)
H (8) - 5 = C (3)
Thus, the 3rd letter is M.

4th letter of each cluster:
C (3) + 6 = I (9)
I (9) + 6 = O (15)
O (15) + 6 = U (21)
U (21) + 6 = 27 ≡ A (1)
Thus, the 4th letter is O.

Combining the resulting letters from each position gives NMMO.