The kinetic energy of a roller coaster is highest at its lowest point. This is when the coaster is moving at its greatest speed due to the conversion of potential energy (from height) into kinetic energy (due to motion).
Without specific details about the roller coaster's layout (like which points A, B, C, or D correlate to the highest or lowest points), I can’t definitively tell you which letter corresponds to that lowest point. However, if you identify which point on the coaster is at the lowest elevation, that would be where the kinetic energy is highest.