Explanation:

- Let's break down the series provided: QESC, ?, QOSM, QTSR, QYSW.

- Observing each position within the letter cluster:

- The first letter in each cluster is "Q."

- The second letters increase by 4 in the alphabet: E (5) -> O (15) -> T (20) -> Y (25).

- The third letters increase by a constant of +4 from S (19) -> S (19).

- The fourth letter: C (3) -> M (13) -> R (18) -> W (23) shows a pattern of +5.

- Analyze the options:

- RLSJ: Offers no consistent pattern with the series.

- QJSH: Matches the increase +4 for second, same for third and +5 for fourth.

- QIRG and RKSI: Do not fit the pattern.

Answer: QJSH