The correct answer is D. All of the above.
Algae, amoeba, and paramecium are all single-celled organisms. They are all eukaryotes, which means that their cells have a nucleus. They are also all heterotrophs, which means that they cannot make their own food and must consume other organisms to survive.
Algae are photosynthetic organisms, which means that they can use sunlight to produce their own food. They are found in both aquatic and terrestrial environments. There are many different types of algae, including green algae, brown algae, and red algae.
Amoeba are protozoans, which are a type of single-celled animal. They are found in both fresh water and salt water. Amoeba move by extending and retracting pseudopods, which are temporary projections of their cytoplasm.
Paramecium are also protozoans. They are found in fresh water. Paramecium have a cell membrane, a cytoplasm, a nucleus, and a contractile vacuole. They move by beating their cilia, which are short hair-like structures.
Single-celled organisms are important members of the food chain. They are a source of food for larger organisms, and they also play a role in the cycling of nutrients.