In an ecosystem, if a fish is feeding on zooplankton, it is a:

The correct answer is Option (3) → Secondary consumer

In a food chain, producers make food; primary consumers eat producers; secondary consumers eat primary consumers. In water, phytoplankton are the producers and zooplankton eat them.

Option (1) Producer — Incorrect. Producers make their own food by photosynthesis; a fish does not.

Option (2) Primary consumer — Incorrect. The zooplankton are the primary consumers here, as they eat phytoplankton.

Option (3) Secondary consumer — Correct. A fish eating zooplankton eats a primary consumer, so it is a secondary consumer.

Option (4) Tertiary consumer — Incorrect. A tertiary consumer eats secondary consumers, e.g. a larger fish eating this fish.