The correct answer is A. Plankton.
Plankton are small organisms that float or drift in the water column. They are not able to swim against the current, and so they are at the mercy of the water currents. This makes them very sensitive to changes in the environment, including pollution.
Plankton are divided into two main groups: phytoplankton and zooplankton. Phytoplankton are photosynthetic organisms, which means that they can produce their own food using sunlight. Zooplankton are heterotrophic organisms, which means that they must eat other organisms to survive.
Plankton are an important part of the food web in aquatic ecosystems. They are a primary food source for many fish and other animals. They are also important in the cycling of nutrients in the water.
Pollution can have a negative impact on plankton populations. Some pollutants can kill plankton directly, while others can disrupt their reproduction or growth. This can lead to a decline in the plankton population, which can have a ripple effect on the entire aquatic ecosystem.
Therefore, plankton are the best indicator of pollution in a water body. They are sensitive to changes in the environment, and they are an important part of the food web.