Which of the following best describes pollution sensitive species?

Pollution-sensitive species are organisms that react adversely to the presence of pollutants in their environment. When these species are present in a water body, it often indicates that the water quality is good and that pollution levels are low. Their sensitivity to changes in water quality makes them valuable indicators for assessing the health of aquatic ecosystems.

These species typically require specific conditions to survive, such as clear water with adequate oxygen levels and low concentrations of toxins or nutrients. Hence, if they are thriving in an environment, it's a strong sign that the water is relatively unpolluted.

The other choices do not accurately represent pollution-sensitive species. Some species can tolerate or thrive in polluted conditions, while others may not necessarily be restricted to freshwater ecosystems. Thus, the presence of pollution-sensitive species reflects a healthy aquatic environment, validating the correctness of the response that describes them as indicators of non-polluted water.