1. Positions: BD = 2, 4; FH = 6, 8; JL = 10, 12; MO = 13, 15.
2. The first three pairs start on an even position; MO starts on the odd position 13.
3. (The gap of 2 is the same in all four, so the gap cannot be the distinguishing feature.)

Answer: MO.

Convert letters to numbers before comparing. When the obvious feature — here the gap of two — is shared by all, look for a second property.

Shortcut. First letters step B → F → J by four places each, but J → M is only three — MO breaks the step. It is also the only group holding a vowel (O).

Common trap. "BD" — as the first pair it looks like the odd starter of the pattern; but all four share the gap of two, and only MO breaks the +4 step.