Choose the correct alternative that will continue the same pattern and replace the question mark (?) in the given letter - cluster series.
HMS, MTB, SBL, ZKW,?

HMS, MTB, SBL, ZKW, ?

Logic:
Alphabetical jumps increase step-by-step for each letter position.

1st letters:
H +5 → M
M +6 → S
S +7 → Z
Z +8 → H  

2nd letters:
M +7 → T
T +8 → B 
B +9 → K
K +10 → U

3rd letters:
S +9 → B 
B +10 → L
L +11 → W
W +12 → I  

So, the missing letter cluster is HUI.

Thus, correct option is (a).