Which letter cluster will replace the question mark (?) to complete the given series?
TDQR, SFNV, ?, QJHD, PLEH

The correct option is RHKZ.

To find the missing letter cluster in the series, let us analyze the pattern of the letters at each corresponding position across the clusters: TDQR, SFNV, ?, QJHD, PLEH.

1st Letter Pattern:
T → S → ? → Q → P
Let us write their numerical positions in the English alphabet:
T = 20, S = 19, Q = 17, P = 16.
The pattern is a constant subtraction of 1:
20 - 1 = 19 (S)
19 - 1 = 18 (R)
18 - 1 = 17 (Q)
17 - 1 = 16 (P)
Therefore, the first letter of the missing cluster is R.

2nd Letter Pattern:
D → F → ? → J → L
Let us look at their numerical positions:
D = 4, F = 6, J = 10, L = 12.
The pattern is a constant addition of 2:
4 + 2 = 6 (F)
6 + 2 = 8 (H)
8 + 2 = 10 (J)
10 + 2 = 12 (L)
Therefore, the second letter of the missing cluster is H.

3rd Letter Pattern:
Q → N → ? → H → E
Let us look at their numerical positions:
Q = 17, N = 14, H = 8, E = 5.
The pattern is a constant subtraction of 3:
17 - 3 = 14 (N)
14 - 3 = 11 (K)
11 - 3 = 8 (H)
8 - 3 = 5 (E)
Therefore, the third letter of the missing cluster is K.

4th Letter Pattern:
R → V → ? → D → H
Let us look at their numerical positions:
R = 18, V = 22, D = 4 (which is 26 + 4), H = 8 (which is 26 + 8).
The pattern is a constant addition of 4:
18 + 4 = 22 (V)
22 + 4 = 26 (Z)
26 + 4 = 30 (which corresponds to 30 - 26 = 4, i.e., D)
4 + 4 = 8 (H)
Therefore, the fourth letter of the missing cluster is Z.

Combining all the letters at their respective positions, we get the cluster: RHKZ.