A food chain in an ecosystem flows from green algae, to freshwater snails, to small fish, and finally to large fish.

The primary consumer would is freshwater snails.

Producers are organisms that produce their own food and not depends on any other organism by the process of photosynthesis such as algae. Primary organisms on the other hand directly depend on the producers for their food and often get killed by secondary consumers.

Freshwater snails are the perfect example of the primary consumer as they get their food and energy from algae and are the primary source of food and energy for small fishes.

currents is the correct answer.