If the RPM is fixed at speed `x`,then that is a particular angle of attack; if the speed changes,then the prop governor(CSU)will adjust the blade angle,so that the same angle of attack is maintained,irrespective of the `size` of the vectors corresponding to that angle of attack.The same applies if the a/c slows down.(accepting that there are transient changes in blade angle).
If the pitch is fixed,RPM can only be maintained by reducing throttle,as stated earlier.
Badly worded question,and rather unfair..