I can try and explain the reasoning (without a diagram, sorry).
The answer should be (A) because all around the track, you are supposed to be undergoing uniform circular motion. The equation you would use is a=v^2/r, and you would assume that at the very top, it just barely touches the tracks, so you can set your normal force (N) to zero. Now the only force on the cart would be gravity, so set acceleration=gravity, and solve for a=v^2/R.
Hopefully that helps.
