Which of these is an autotroph? A.
mackerel
B.
squid
C.
mollusks
D.
fungi
E.
phytoplankton

E.

phytoplankton

Explanation:

The defenition of Autotroph is: "an organism that is able to form nutritional organic substances from simple inorganic substances such as carbon dioxide." (Oxford Dictionaries)  

As Britannica says, "[...]Like land vegetation, phytoplankton uses carbon dioxide, releases oxygen, and converts minerals to a form animals can use."

Then Wikipedia (I know, not the best source, but it works) says, "Phytoplankton (/ˌfaɪtoʊˈplæŋktən/) are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems."

So, Phytoplankton is an autotroph.

when it is at an intermediate population density