Choose the missing terms out of the given alternatives. AB, DEF, HIJK, ?, STUVWX

The correct answer is $\boxed{\text{LMNO}}$.

The sequence is a simple letter substitution cipher, with each letter being replaced by the letter three places after it in the alphabet. So, A becomes D, B becomes E, and so on.

Therefore, the missing term is LMNO, which comes after HIJK in the alphabet.

The other options are incorrect because they do not follow the same pattern. For example, LMNOP would be the next term in the sequence if each letter were replaced by the letter four places after it in the alphabet. However, this is not the case in the given sequence.