In the series AB, EDC, FGHI, ....?...., OPQRST, the missing term is

Concept: Letter-series items built from blocks of growing length often carry two independent rules at once: the block size grows by a fixed step while consuming consecutive alphabet letters, and the internal order of the letters in each block alternates between a forward run and its exact reverse.

1. AB is the first 2 letters of the alphabet (A, B), written forward.

2. EDC is the next 3 letters (C, D, E), written in reverse.

3. FGHI is the next 4 letters (F, G, H, I), written forward again.

4. The missing block must hold the next 5 letters, J, K, L, M, N -- and following the forward/reverse alternation, this block must be written in reverse: N, M, L, K, J.

5. OPQRST is the next 6 letters (O, P, Q, R, S, T), written forward -- consistent with the alternation resuming forward right after a reversed block.

Cross-check: Two independent features both hold once the missing block is read as N, M, L, K, J: the block sizes form a clean 2, 3, 4, 5, 6 progression, and the forward/reverse alternation (forward, reverse, forward, reverse, forward) is unbroken across all five terms.

Result: The missing term is N, M, L, K, J.