Pattern:
To find the missing term in the series, we need to identify the pattern followed in the given series.
- In the series AB, DEF, HIJK, each term consists of consecutive letters of the alphabet.
- The first term, AB, consists of the first two letters of the alphabet.
- The second term, DEF, consists of the fourth, fifth, and sixth letters of the alphabet.
- The third term, HIJK, consists of the eighth, ninth, tenth, and eleventh letters of the alphabet.

Explanation:
Based on the pattern, we can determine the missing term in the series.

- The first term, AB, consists of the first two letters of the alphabet.
- The second term, DEF, consists of the fourth, fifth, and sixth letters of the alphabet.
- The third term, HIJK, consists of the eighth, ninth, tenth, and eleventh letters of the alphabet.

Following the same pattern, the missing term should consist of consecutive letters starting from the thirteenth letter of the alphabet.

Options:
a) QRSTU
b) LMNOP
c) LMNO
d) MNOPQ

Analysis of Options:
a) QRSTU: This option consists of the seventeenth, eighteenth, nineteenth, twentieth, and twenty-first letters of the alphabet. This does not follow the pattern observed in the given series.
b) LMNOP: This option consists of the twelfth, thirteenth, fourteenth, fifteenth, and sixteenth letters of the alphabet. This matches the pattern observed in the given series.
c) LMNO: This option consists of the twelfth, thirteenth, fourteenth, and fifteenth letters of the alphabet. This does not match the pattern observed in the given series.
d) MNOPQ: This option consists of the thirteenth, fourteenth, fifteenth, sixteenth, and seventeenth letters of the alphabet. This matches the pattern observed in the given series.

Conclusion:
Based on the analysis, we can conclude that the missing term in the series AB, DEF, HIJK, ______, STUVWX is MNOPQ (option d).