Context: Below is a food web from an ocean ecosystem in Monterey Bay, off the coast of California.
A food web models how the matter eaten by organisms moves through an ecosystem. The arrows in a food web represent how matter moves between organisms in an ecosystem.
Which of the following organisms is the primary consumer in this food web?
A. kelp
B. sea otter
C. phytoplankton
D. zooplankton
Determine the correct option from the provided choices and reply with its corresponding letter.

D