Which letter-cluster will replace the question mark (?) to complete the given series?

PQST, TOVS, ?, BKBQ, FIEP

The correct option is XMYR.

To find the letter-cluster that replaces the question mark (?), let's analyze the pattern by comparing the letters in the corresponding positions of each cluster in the series:
PQST, TOVS, ?, BKBQ, FIEP

Let's convert each letter to its position in the English alphabet (A = 1, B = 2, ..., Z = 26) to find the logic:

First Letter of each cluster:
P (16) → T (20) → ? → B (2) → F (6)
Looking at the differences between the positions:
16 + 4 = 20 (T)
20 + 4 = 24 (X)
24 + 4 = 28. Since there are 26 letters in the alphabet, 28 corresponds to 28 - 26 = 2 (B).
2 (B) + 4 = 6 (F)
The pattern for the first letter is adding 4 at each step. Thus, the first letter is X (24).

Second Letter of each cluster:
Q (17) → O (15) → ? → K (11) → I (9)
Looking at the differences between the positions:
17 - 2 = 15 (O)
15 - 2 = 13 (M)
13 - 2 = 11 (K)
11 - 2 = 9 (I)
The pattern for the second letter is subtracting 2 at each step. Thus, the second letter is M (13).

Third Letter of each cluster:
S (19) → V (22) → ? → B (2) → E (5)
Looking at the differences between the positions:
19 + 3 = 22 (V)
22 + 3 = 25 (Y)
25 + 3 = 28. Which corresponds to 28 - 26 = 2 (B).
2 + 3 = 5 (E)
The pattern for the third letter is adding 3 at each step. Thus, the third letter is Y (25).

Fourth Letter of each cluster:
T (20) → S (19) → ? → Q (17) → P (16)
Looking at the differences between the positions:
20 - 1 = 19 (S)
19 - 1 = 18 (R)
18 - 1 = 17 (Q)
17 - 1 = 16 (P)
The pattern for the fourth letter is subtracting 1 at each step. Thus, the fourth letter is R (18).

Combining all the letters at the third position gives us XMYR.