Which of the following does not occupy the bottom of an energy pyramid? a. phytoplankton b. trees c. grass d. zooplankton

ZOOPLANKTON does not occupy the bottom of an energy of pyramid.
An energy of pyramid refers to a diagrammatic representation of the flow of energy from one trophic level to another in a food web. When organisms in one trophic level eat other organisms, transfer of energy occur between the trophic levels. The bottom of an energy of pyramid is occupied by autotrophs, they are also called producers. Autrotrophs are capable of producing their own food and all other organisms in the food web depend on them for food. From the options given in the question above, all of them are autotrophs with the exception of zooplankton, which is a heterotrophic organism. 

Energy pyramid is the graphical model that shows the flow of energy in a community. The levels of the pyramid shows different microorganisms in the food chain. One of the many organisms that occupy the bottom of the pyramid is letter D. zooplankton.