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The place an organism attains in a food web is called its trophic level. There is a maximum of three to four trophic levels in a food chain. The first level is producers, the second level is primary consumers, the third level is secondary consumers, and the fourth level is tertiary consumers.

In the environment, cyanobacteria are identified asthe most significant primary producers. Phytoplankton includes cyanobacteria, also known as blue-green algae. They are the leading producers in the sea.

Thus, ‘cyanobacterium’ and ‘primary producer’ are correctly paired.

Therefore, the given option is incorrect.

Grasshoppers are the main consumers because they consume the producers, which are plants or grass. As they eat grass, they are one trophic level higher than grass and are primary consumers, which is the second trophic level.

Thus, ‘grasshoppers’ and ‘primary consumers’ are correctly paired.

Therefore, the given option is incorrect.

Zooplankton is the principal consumer at the bottom of the aquatic food web; hence, they feed on phytoplankton, which obtains their energy directly from the sun.

Thus, ‘zooplankton’ is incorrectly paired with ‘primary producer’.

Therefore, the given option is correct.

Detritivores are types of decomposers. Animals that break down and devour dead or rotting stuff are known as decomposers. In the forest, fungi eat the decaying matter. Thus, fungi are decomposers.

Thus, ‘fungi’ is correctly paired with ‘detritivores’.

Therefore, the given option is incorrect.