For this aircraft In the lower speed ranges you would expect the THP available to be a nearly flat line, although I admit a constant speed prop would give a flatter trace.
I think that you are mixing up Thrust and THP.
THP = Thrust X TAS, so all aircraft propulsion systems produce zero THP when TAS is zero.
A constant speed prop will maintain thrust up to higher airspeeds, so the thrust line will be flatter.
But THP = Thrust X TAS, so a flat thrust line will give a THP line which increases with increasing TAS.
A constant speed prop will extend the THP curve further to the right.
This will increase the speed for best ROC.
For a very basic piston prop VX will be a smidgin less than Vmp and Vy will be a smidgin more than Vmp.
For higher powered aircraft, particularly those with constant speed props Vy will be closer to Vmd.
The correct only correct answer in this question is
c) is bteween 95 and 125 kt