(a) The roller coaster is brought to the highest point.
(b) The potential energy of the coaster decreases, but the speed and the kinetic energy increase.
(c) After passing through the «valley», the height of the coaster increases, but the speed and the kinetic energy decrease.
(d) Due to frictional lost, the mechanical energy of the coaster has decreased, so the second «hill» has to be lower than the first one.