My suggested answers: assuming that point C is on the same level if not lower than A.
a) Energy is lost to the surrounding as it travelled along the track.
b)
i) Adjust the flexible track such that A is is higher
ii) Apply some force to the car instead of releasing it
c) (Gravitational) Potential energy and kinetic energy
Because car is still moving (kinetic) and it's above the ground (gravitational potential energy)

Because I am unsure whether you learnt the term gravitational, I placed a bracket to make it optional.
Hope this helps.

Thank you so much

Can we reduce the diameter of the loop by pulling at point d? So that the toy car can reach at point c.

But question stated that distance between point C and B is fixed, I think the option you give may not be possible

Thanks for the clarification