Select the group that replaces the question mark in the letter analogy: ? : GIK :: NPR : TVX

The completed pair fixes the rule: N (14), P (16) and R (18) each move on 6 places to give T (20), V (22) and X (24). Stepping back 6 from G (7), I (9) and K (11) gives A (1), C (3) and E (5), that is ACE. 'BDF' is the sharp trap, since it does reach GIK — but only with a step of 5, and 'CEG' only with a step of 4, so both contradict the step of 6 the given pair lays down. 'ADG' is not even evenly spaced, and moving it on 6 would give GJM.