All of the schools have learned much more about the JAA CQB since my earlier post in this thread. It is now widely known that the answer to this question is that pitch angle decreases in a constant mach descent. So if the objective is to get the mark in the examination, then "decreases" is the one to pick.
A similar problem concerned the question of why step climbs are carried out. The options included "to comply with ATC requirements" (or words to that effect), and "to maximise range"(or words to that effect). Different schools argued for different options. It is now known that the JAA answer is "to fly as close as possible to the optimum altitude as aeroplane mass reduces".
These answers have been proved to be correct (or at least to be the ones that get the marks in the exam) by various students who have scored 100% in the Performance exam.