Final answer:
Milo was studying herbivores, which are primary consumers in the food chain that consume plants or other producers.
Step-by-step explanation:
The link in the food chain that Milo was studying can be inferred by looking at the context provided, which hints at the overeating by herbivores causing depletion of primary producers. This would suggest that herbivores are being studied, as the question indicates a concern about the effects of their consumption on the food chain. Herbivores are organisms that eat producers (like plants or phytoplankton) and are considered primary consumers within an ecosystem. Therefore, the most appropriate answer to the question 'What link in the food chain was Milo studying?' would be c) Herbivores.