What should come in place of the question mark (?) in the given series based…

Concept

In a positional letter series built from three-letter groups, each of the three letter-slots (1st, 2nd, 3rd position within a group) moves through the alphabet by its own fixed step from one group to the next. Solve each slot's column independently first, then combine the three results into the missing group.

Application

1. 1st letters of each group: O, N, M, L -> alphabet positions 15, 14, 13, 12 -> each one less than the previous -> next value is 11, i.e. K.

2. 2nd letters of each group: H, G, F, E -> alphabet positions 8, 7, 6, 5 -> each one less than the previous -> next value is 4, i.e. D.

3. 3rd letters of each group: L, K, J, I -> alphabet positions 12, 11, 10, 9 -> each one less than the previous -> next value is 8, i.e. H.

4. Combine the three slot results in order (1st, 2nd, 3rd) to form the missing group: K, D, H -> KDH.

Cross-check

Re-checking the step size across all four given groups confirms every one of the three slots decreases by exactly one letter each time (O-N-M-L, H-G-F-E, L-K-J-I), so the same step applies going into the fifth group, confirming the missing group is KDH.