A Sponge is an animal. It belongs to the phylum Porifera and is classified as a multicellular organism. Sponges are unique animals because they lack true tissues and organs. Instead, their bodies are made up of specialized cells called choanocytes, which filter and circulate water through their bodies. Sponges are considered the simplest form of animal life.

Option 1, A fungus, is incorrect. Fungi belong to a different kingdom and have distinct characteristics, such as cell walls made of chitin.

Option 2, A fossil, is incorrect. Fossils are the preserved remains or traces of organisms that lived in the past. Sponges can form fossils, but they are not exclusively fossils.

Option 3, A plant, is incorrect. Plants belong to a different kingdom and have distinct characteristics, such as the ability to photosynthesize.

The correct answer is option 4, An animal.