How can the airspeed increase at fixed rpm?
The pilot could descend.
It isn't supposed to be a realistic scenario....... "it wouldn't do that".
There are many things which are not trained at each level of training for various licenses - because they are not realistic, and the flight of a pilot at that level "would not do that". It's a stretch to examine for things which are not realistic.
If the question is intended to probe the candidate's understanding of an unrealistic scenario (which probably was never trained),
all of the information required to make a determination should be provided. In the case of this question, as written, the information required to formulate an understanding and answer correctly was incomplete.
almost certainly from the fixed pitch syllabus
So the question may be
almost properly worded then....