that (Max Excess THP) will occur at the highest TAS,
OK let's test that theory. We accelerate to our maximum possible speed where power available = power required and thrust equals drag. We now have zero excess thrust and zero excess THP. So our best climb angle is zero and our best roc is zero.
We will not get best excess THP by flying at max TAS. We will get best excess THP by flying at the speed at which power available minus power required is a maximum. The value of this speed depends on the shape of the power available and power required curves.
Most texts show prop power available as a hump shape starting at zero when TAS equals zero. All power available curves that I have ever seen look a bit like a Nike tick.
I believe that you are an ex RAF fighter pilot. If so you probably have some old copies of the AP3456. If that is the case I suggest you look at them. You will not see a horizontal line for prop power available. If you do not have the 3456 then pretty well any similar text will do. They all show the curves as I have decsribed them.
but constant power implies that the product of T*V remains constant,..
But prop THP is not at all constant, it is a hump shape, so the implication is of no significance.
.although that is an idealized situation
It is so idealized that it bears no resemblance to reality.
If you look at those diagrams in the 3456 (I'm looking at them now) (Yes I stole one as well) you will see that VY prop is more than Vmp, but significantly less than Vmd.
Increasing engine power and adding a constant speed prop will extend the THP curve out the right. This will take Vy closer to Vmd. Now if we go back to the original question we will see that this gives us option c as the correct answer.
TAS must increase for constant power; of course the power setting will be lower for level flight than climbing flight,..but the maximum excess thrust in the case stated still should occur at Vimd as both thrust and drag decrease together in a climb