Answer: D. Zooplankton.
The bottom of the food pyramid consists of producers. A producer is a term for anaerobic and/or plants that can create their own food using photosynthesis and other means. The reason why zooplankton is not on the bottom of the pyramid is because zooplankton is a consumer. Zooplankton essential gains it's energy from feeding off of phytoplankton, not by creating it's own food source.
Thus, the answer is D