...which one of the following is a pair of areas neither of which could be reduced
(C) L, N
(D) L, P
After putting M and R in the reduce group (and inferring that W follows), I was able to infer that L and N could not both be in the reduce group. I would have easily picked (C), but then I looked at rule #3, and realized that both L and P cannot both be in the reduced group, so I got stuck. Both (C) and (D) give letters that both cannot be in the reduced group.