Find the odd one out?

Concept

In a letter-series ‘odd one out’ problem, each group is built on a fixed rule: the gaps (number of alphabet positions) between consecutive letters follow one common pattern. Four groups obey the same gap pattern; the single group that breaks it is the exception.

Application

Replace each letter by its position in the alphabet (A=1, B=2, …) and read the differences between successive letters. The four matching groups all follow the gap pattern +2, +1, +3:

• ACDG → 1, 3, 4, 7 → gaps +2, +1, +3

• EGHK → 5, 7, 8, 11 → gaps +2, +1, +3

• JLMP → 10, 12, 13, 16 → gaps +2, +1, +3

• LNOR → 12, 14, 15, 18 → gaps +2, +1, +3

Now test QSTU → 17, 19, 20, 21 → gaps +2, +1, +1. The first two steps match (+2, +1), but the final step is +1 instead of +3.

Cross-check

If QSTU were to follow the same rule, its last letter would have to be 20 + 3 = 23, i.e. W, giving QSTW — not QSTU. Since every other group keeps the +3 final jump and only QSTU does not, QSTU is the odd one out.