In marine food chains, the are microscopic organisms called phytoplankton.
a. consumers
b. producers
c. herbivores
d. heterotrophs

~Hello there!

Your question: In marine food chains, the are microscopic organisms called phytoplankton. 

Your  In marine food chains, the heterotrophs  are microscopic organisms called phytoplankton. 

The answer is option D.

Note: Heterotrophs can be referred to BOTH single-celled and as well as multi-celled organisms.

Any questions ^?

Happy Studying!

I’d say a : water molecules