As with (far too many) JAR questions, you must make a number of assumptions. Firstly you must assume that you are gliding, so speed control is by means of changes in pitch atitude only (you cannot simply open or close the throttles).
Having made the above assumption you can find out what happens to CAS by using the C-T-M sketches. When descending at constant mach, the M line will be vertical, with the C and T lines converging on it as altitude decreases. This means that as you descend at constant Mach, your CAS will increase.
But increasing CAS means increasing dynamic pressure. If your angle of attack remains constant, increasing dynamic pressure will give increasing drag, so the aircraft will decelerate. But to have constant Mach, the aircraft must accelerate to increase its CAS. So you must push the nose down in order to reduce angle of attack and drag. This will enable you to increase CAS in order to maintain constant Mach.
The next problem comes in deciding whether pushing the nose down means that pitch angle is increasing or decreasing. There is no universally agreed answer on this point. Some schools teach that pushing the nose down below the horizon is increasing pitch, while others teach that it is decreasing pitch.
At the end of the day you must choose one or the other. But you should also appeal on the basis that there is no universally agreed definition of whether pitch is increasing or decreasing in this situation. This problem has been going on for a very long time but the CAA appear to be unable to recognise it.
The problem could of course be solved by a single e-mail message from the CAA to all the schools, stating what the JAA believe happens to pitch angle when the nose is pushed further down or pulled up when it is already pointing below the horizon.