Which of the following letter-clusters will replace the question mark (?) in the given series?
IJKL, MNOP, QRST, ?, YZAB

Let's observe the series of letter clusters:
1st cluster: IJKL (continuous alphabetical sequence starting from I)

2nd cluster: MNOP (starts right after L, continuous sequence)

3rd cluster: QRST (starts right after P, continuous sequence)

Following this continuous pattern:

The 4th cluster must start right after T, which is U.

A 4-letter continuous sequence starting from U is UVWX.

Let's cross-verify with the next cluster:

After X, the next letter is Y, and the sequence is YZAB, which matches the final cluster.

Hence, the correct cluster is UVWX.