A biomass pyramid demonstrates the idea of energy loss down the food chain, therefore A is correct;
Biomass is biological material used as food or source of energy, therefore B is correct as well. But this term can also be used to define the mass of living organisms in a given ecosystem at a given time. Therefore, D is correct too.
As the pyramid demonstrates the loss of energy, it starts with the producers (that has more energy) at its basis, and the amount of energy loss shows the difference from the producers in the very bottom of the pyramid to the primary consumers (which has less energy than the producers), right above the producers. The same keeps happening as we check the levels from the bottom to the top, and the secondary consumer will have less energy than the primary consumer, as the tertiary consumer will have less energy than the secondary consumer, and so this goes on. Therefore, the tertiary consumers will have lower energy levels than the producers, and C is incorrect.