The correct answer is option 4: Planaria. Planaria is a free-living animal, meaning it does not depend on another organism for its survival. This flatworm is found in freshwater habitats such as ponds and streams. It has a distinct head and tail region, and its body is covered in cilia, small hair-like structures that help the organism move. Planaria is able to feed on small organisms and organic matter in its environment.

Option 1: Liver fluke is not a free-living animal. It is a parasite that infects the liver of animals, such as sheep and cattle.

Option 2: Wuchereria is also not a free-living animal. It is a parasite that causes the disease called lymphatic filariasis in humans.

Option 3: Plasmodium is a parasite that causes malaria. It is not a free-living animal as it requires a host, typically a mosquito, to complete its life cycle.

Therefore, the correct option is 4: Planaria, a free-living animal found in freshwater environments.