Regarding the second question, there appears to be a similar version of this question, as follows:
As you can see, this question uses radius instead of diameter. The working for this version of the question is as follows:
A | By conservation of energy,
GPE of model car at top of track = GPE + KE of model car at top of loop
⇨
⇨ |
The radius, being equal to half the diameter, means that 2.5r is equal to 5D/4 (i.e. 1.25D) making the answer to the second question (A).
I hope this helps!