Understanding the Series
To identify the missing term in the series: AB, DEF, HIJK, ?, STUVWX, we need to analyze the pattern in the sequences of letters.

Breakdown of Patterns
- **AB**: This consists of 2 letters.
- **DEF**: This consists of 3 letters.
- **HIJK**: This consists of 4 letters.
- **?**: The missing term should ideally consist of 5 letters.
- **STUVWX**: This consists of 6 letters.

Identifying the Missing Term
Following the pattern of increasing letters:
- The first term has 2 letters (AB).
- The second term has 3 letters (DEF).
- The third term has 4 letters (HIJK).
- Thus, the fourth term should have 5 letters.

Possible Options
Now, let's evaluate the provided options:
- a) LMNO (4 letters)
- b) LMNOP (5 letters)
- c) MNOPQ (5 letters)
- d) QRSTU (5 letters)
Among these options, both b) LMNOP and c) MNOPQ consist of 5 letters.

Choosing the Correct Option
However, to maintain continuity in the alphabet:
- **LMNOP** follows immediately after **HIJK** in the sequence.
- **MNOPQ** also continues in alphabetical order but is not the direct consecutive sequence.
Thus, the correct missing term that fits the pattern is **c) MNOPQ**, as it follows the alphabetical order and maintains the series' increasing letter count.

Conclusion
Therefore, the missing term in the series is **MNOPQ (option c)**, which aligns with the pattern of increasing letter count and maintains continuity in the alphabet.