The correct answer is A. Protozoa.
Protozoa are a diverse group of single-celled eukaryotes. They are found in all environments, including water, soil, and the human body. Protozoa are classified based on their mode of locomotion, which can be by pseudopodia, flagella, or cilia.
Amoeba is a genus of protozoa that are characterized by their ability to change shape. They move by extending pseudopodia, which are temporary projections of the cell membrane. Amoeba are found in freshwater and soil. They are an important part of the food chain, as they are a source of food for other animals.
Annelida are a phylum of segmented worms. They are found in both freshwater and marine environments. Annelida are characterized by their segmented body, which is made up of a series of rings. Each ring contains a set of organs, including a digestive system, a circulatory system, and a nervous system.
Porifera are a phylum of animals that are also known as sponges. They are found in marine environments. Porifera are characterized by their porous body, which is made up of a network of canals. The canals allow water to flow through the body of the sponge, bringing food and oxygen and removing waste products.
Platyhelminthes are a phylum of flatworms. They are found in both freshwater and marine environments. Platyhelminthes are characterized by their flat body, which is bilaterally symmetrical. They have a digestive system with a mouth and an anus.
In conclusion, the correct answer is A. Protozoa.