Explanation

Planktonic forms (phytoplankton) are producers. Fish that eat them are primary consumers. However, the question asks for the 'first fish' eating planktonic forms, which implies the trophic level of the fish relative to the plankton. If the plankton includes zooplankton (primary consumers), then a fish eating zooplankton is a secondary consumer. In many ecological contexts, 'planktonic forms' often refers to zooplankton as the first food source for small fish, making them secondary consumers in the broader chain starting from phytoplankton. Given standard MCQ patterns, D is likely intended.