Take intoaccount that for points A, D, and B, almost all the mechanical energy is potential gravitational energy. For the lowest point the potential energy is least and then the kinetic energy is maximum.
Then, you can conclude that for point C the kinetic energy is maximum.
If drag and friction forces are included, then, for point E (that is, after the roller coaster is traveled all the trajectory shown) part of the mechanical energy has been dissipated.
For point E the mechanical energy is minimum.