Paul,
The initial mistake they made was to overlook the fact that prop aircraft and jet aircraft behave very differently. In some of the old questions there were two correct answers, one for props (Vx increases Vy decreases) and one for jets (Vx constant and Vy decreasing). Unfortuanately they did not specify the aircraft type. The problem is made even more complicated by the fact that for some low speed fixed pitch prop aircraft Vx decreases while Vy decreases even faster!
In the question above you cpoulfd pick option (a) or (d) each of which is correct for some props and the jet answer is not there.