Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.
OTE, PUF, QVG, RWH, ?

Let's look at the progression for each letter position independently:
1st letter:

O→P→Q→R→S (+1 shift)

2nd letter:

T→U→V→W→X (+1 shift)

3rd letter:

E→F→G→H→I (+1 shift)

Putting them together, the next cluster in the series is SXI.