The correct answer is (a) Algae.
Algae are a type of plant that can live in both water and soil. They are very sensitive to changes in their environment, and can be used to indicate the presence of pollution. For example, algae blooms can occur when there is too much nitrogen or phosphorus in the water. These blooms can block sunlight and suffocate other plants and animals, and can also produce toxins that can be harmful to humans.
Fungi are not plants, but they are also sensitive to changes in their environment. They can be used to indicate the presence of pollution, but they are not as reliable as algae. Lichens are a type of fungus that grows in association with algae. They are also sensitive to changes in their environment, but they are not as commonly used as algae to indicate pollution. Ferns are a type of plant that is not sensitive to pollution. They are not a good indicator of pollution.
In conclusion, algae are the best indicator of pollution among the options given.