One of these four letter groups is put together differently from the rest. Choose the odd one out: ACE, GIK, MOQ, PRT.

In ACE the position values are 1, 3, 5, in GIK they are 7, 9, 11 and in MOQ they are 13, 15, 17, so every letter in those three groups stands at an odd-numbered position. PRT runs 16, 18, 20, all even. The gap gives nothing away, since all four groups skip one letter each time; only the odd or even positions separate them.