Hi CKK! Thanks for your question
The term "larger sea animals" in answer choice (C) is actually referring to the fish and seabird populations mentioned in the first sentence of the stimulus (the conclusion), not the relative size of zooplankton and phytoplankton.
If we were to prephrase the answer to this question, it would be something like the following: it is a premise that supports the conclusion that declines in fish and seabird populations are inevitable due to global warming, by explaining how the effect of global warming on phytoplankton will indirectly affect fish and seabird populations through the food chain. Answer choice (C) matches that prephrase.
In general, make sure you keep in mind the conclusion on any Method-Argument Part question to ensure that you understand how the sentence they are asking you about relates to the conclusion of the argument.
I hope this helps, and let me know if you have any other questions!