Odd man out:

In an alphabet 'odd one out' series, each option is a group of three letters formed by moving forward through the alphabet by a fixed number of positions between consecutive letters. Three of the four groups share the same step size; the option whose step size changes partway through is the one that breaks the pattern.

1. FIL: F is the 6th letter, I is the 9th letter, L is the 12th letter — steps of +3 and +3.

2. RUX: R is the 18th letter, U is the 21st letter, X is the 24th letter — steps of +3 and +3.

3. ILO: I is the 9th letter, L is the 12th letter, O is the 15th letter — steps of +3 and +3.

4. LOQ: L is the 12th letter, O is the 15th letter, Q is the 17th letter — steps of +3 and +2.

FIL, RUX, and ILO each move forward by exactly 3 positions twice, so they follow one uniform rule. LOQ moves forward by 3 positions and then only 2 positions, so it does not match that uniform rule — making LOQ the odd one out.