The four letter groups below are built to one pattern, except one. Choose the odd one out: ACF, DFJ, BDG, CEH.

In ACF the places are 1, 3, 6, in BDG they are 2, 4, 7 and in CEH they are 3, 5, 8, so each group opens with a gap of 2 and then widens to a gap of 3. DFJ runs 4, 6, 10: the first gap is 2 as it should be, but the second is 4. Every group grows by widening steps, so only the size of that second step gives the odd group away.