Which letter cluster will replace the question mark (?) to complete the given series?
RDLS, VKBF, ?, DYHF, HFXS

The correct answer is ZRRS.

To find the missing letter cluster in the series, let us analyze the pattern followed by each letter position in the clusters: RDLS, VKBF, ?, DYHF, and HFXS.

Let us write down the alphabetical positions of the letters (A = 1, B = 2, ..., Z = 26):

1st Letter Pattern: R → V → ? → D → H
R = 18
V = 22
D = 4 (or 30)
H = 8 (or 34)
The difference between consecutive letters is +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)
So, the first letter is Z.

2nd Letter Pattern: D → K → ? → Y → F
D = 4
K = 11
Y = 25
F = 6 (or 32)
The difference between consecutive letters is +7:
4 + 7 = 11 (K)
11 + 7 = 18 (R)
18 + 7 = 25 (Y)
25 + 7 = 32 (which corresponds to 32 - 26 = 6, i.e., F)
So, the second letter is R.

3rd Letter Pattern: L → B → ? → H → X
L = 12
B = 2 (or 28)
H = 8
X = 24
The difference between consecutive letters is -10:
12 - 10 = 2 (B)
2 - 10 = -8 (which corresponds to 26 - 8 = 18, i.e., R)
18 - 10 = 8 (H)
8 - 10 = -2 (which corresponds to 26 - 2 = 24, i.e., X)
So, the third letter is R.

4th Letter Pattern: S → F → ? → F → S
S = 19
F = 6
The pattern alternates between subtracting and adding 13:
19 - 13 = 6 (F)
6 + 13 = 19 (S)
19 - 13 = 6 (F)
6 + 13 = 19 (S)
So, the fourth letter is S.

Combining the letters from each step, we get the letter cluster ZRRS.