Which of the following letter-clusters will replace the question mark (?) in the given series?
XI, EE, LB, SZ, ?

Let's split the letter clusters into first and second letters and trace their progressions:
First letters: X, E, L, S, ?
Positional values:

X = 24

E = 5 (or 26+5=31)

L = 12

S = 19

Let's find the differences:

24 to 31 →+7

5 to 12 →+7

12 to 19 →+7
The pattern is a constant addition of +7.
Next letter: 19+7=26, which is Z.

Second letters: I, E, B, Z, ?
Positional values:

I = 9

E = 5

B = 2

Z = 26 (or 0)

Let's find the differences:

9 to 5 →−4

5 to 2 →−3

2 to 26 →−2
The pattern of differences is decreasing by 1: −4,−3,−2,…
The next difference must be −1.
26−1=25, which is Y.

Combining them gives ZY.