The correct answer is option 2: Mollusc. Invertebrates are animals that do not have a backbone, and molluscs are the largest group of invertebrates. They include creatures like snails, squids, octopuses, and clams. Molluscs can vary greatly in size, from tiny species to gigantic ones like the giant squid, which can reach lengths of up to 43 ft. Some molluscs, like clams, have hard shells while others, like squids and octopuses, have soft bodies. Molluscs have a diverse range of body structures and lifestyles, and they can be found in various habitats, including oceans, freshwater systems, and land. Overall, the largest invertebrate is a mollusc, making option 2 the correct answer.