The correct option is D 1 - Insects
2 - Molluscs
3 - Crustaceans
4 - Other animal groups
In the given diagram (1) is Insecta, a class of phylum Arthropoda, which has the most number of species among all animals. Phylum Mollusca is the second largest group of animals. The third largest group of invertebrates is the Crustaceans, another class of phylum Arthropoda. (4) is other animal groups.