a) Algae is one phytoplankton found in the sea. It is a producer and it makes its own food by photosynthesis i.e. using water, carbon dioxide in the presence of sunlight and chlorophyll.

b) Crustacea, krill, copepods, protozoa, tadpole are few examples of zooplankton in the sea.

c) X could be small fishes; fishes feed on zooplankton in aquatic food chain.

d) Y could be a carnivorous fish or it can also be a man. Both, Carnivorous fish and humans feed on small fishes.

e) i) Primary consumer in the above food chain are zooplankton as they feed on phytoplankton like algae.

ii) Tertiary consumer in the above food chain can be carnivorous fish or man as they eat small fishes i.e. secondary consumers.