When a big fish eats a small fish which eats water fleas supported by phytoplankton, water fleas are:
A. Producers
B. Primary consumers
C. Secondary consumers
D. Top consumers

Hint: Photosynthetic organisms like phytoplanktons and plants are producers in an ecological pyramid. They are the pyramid's initial trophic level. Producers eat the major consumers. They are the pyramid's second trophic level. Primary consumers are eaten by secondary consumers. They are the pyramid's third trophic level. The highest consumers, who occupy the fourth trophic level, are at the summit of the pyramid.

Complete answer:
Let us solve this question relating it with the food chain concepts.
Option:A – Water fleas cannot be producers as in an aquatic food chain phytoplankton are the producers as they undergo photosynthesis.
Thus, Option:A is not correct.
Option:B – Water fleas are the first consumers of the producers which are phytoplankton so they are called primary consumers. Primary consumers are mostly herbivores.
Thus, Option:B is the correct answer.
Option:C – the secondary consumers are small fishes which feed on water fleas which are primary consumers.
So, Option:C is also not correct.
Option:D - Big fishes are the top consumers of the food chain illustrated in the question as no other organism feeds on them. So, water fleas cannot be top consumers as well.
Thus, Option:D is also not correct.

Therefore, option B. Primary consumers is the correct answer

Note:
A food chain is a logical succession of organisms that begins with producers and ends with decomposers. A food web is a network of interconnected food chains. A food chain has only one path, but a food web has numerous pathways. We may learn about the relationships between species by looking at the food chain. The food web and food chain are essential components of this ecosystem.