C. if q is negatively charged, there is: FN at the lowest point? Mg = mv2r, the solution is: FN=mg+mv2r, so the pressure on the track can't be zero, so C is wrong;
D if q is a positive charge, at the lowest point: fn+mg = mv2r, the solution is: FN+mg=mv2r? Mg, so FN can be less than gravity, so d is wrong.
So choose one