The correct answer is B. Zooplankton.
Zooplankton are small animals that live in the water column and are a major food source for fish and other aquatic animals. They are similar to cows in grasslands in that they are both primary consumers, meaning that they eat plants.
Nekton are animals that can swim freely in the water, such as fish, squid, and dolphins. They are not equivalent to cows in grasslands because they do not eat plants.
Phytoplankton are tiny plants that live in the water column. They are the primary producers in aquatic ecosystems, meaning that they are the basis of the food chain. They are not equivalent to cows in grasslands because they are not animals.
Benthos are organisms that live on or near the bottom of the ocean or other bodies of water. They include animals, plants, and bacteria. They are not equivalent to cows in grasslands because they do not live in the same environment.