We have that a blackbody radiator either constantly absorbs energy or constantly emits energy, depending on its surroundings. In this case, the energy is continuously and smoothly decreasing, thus it cannot be like B and C.
The energy loss or gain is also monotonous, it has the same direction; a radiator cannot gain energy at some point and then lose some. Hence, it does not resemble a wave either. The most appropriate model is the ramp. Energy is constantly emitted to surroundings and it decreases monotonically.
