Which letter-cluster will replace the question mark (?) to complete the given series?
UTHS, VRKO, ?, XNQG, YLTC

The correct option is WPNK.

Let's analyze the pattern by observing the position of each letter in the alphabetical series for the given letter-clusters:

1st letter of each cluster:
$\text{U (21)}\to \text{V (22)}\to \text{W (23)}\to \text{X (24)}\to \text{Y (25)}$
Pattern: Each letter increases by +1 position in the alphabet ($21+1=22$, $22+1=23$ which is W).

2nd letter of each cluster:
$\text{T (20)}\to \text{R (18)}\to \text{P (16)}\to \text{N (14)}\to \text{L (12)}$
Pattern: Each letter decreases by 2 positions in the alphabet ($20-2=18$, $18-2=16$ which is P).

3rd letter of each cluster:
$\text{H (8)}\to \text{K (11)}\to \text{N (14)}\to \text{Q (17)}\to \text{T (20)}$
Pattern: Each letter increases by 3 positions in the alphabet ($8+3=11$, $11+3=14$ which is N).

4th letter of each cluster:
$\text{S (19)}\to \text{O (15)}\to \text{K (11)}\to \text{G (7)}\to \text{C (3)}$
Pattern: Each letter decreases by 4 positions in the alphabet ($19-4=15$, $15-4=11$ which is K).

Combining all the letters together, the letter-cluster that replaces the question mark (?) is WPNK.