Crabs belong to the phylum Arthropoda. Arthropoda is the largest phylum in the animal kingdom and includes creatures such as insects, spiders, and crustaceans like crabs.
Option 1: Mollusca is incorrect. Mollusca is a different phylum that includes animals like clams, snails, and octopuses.
Option 2: Cnidaria is incorrect. Cnidaria is another phylum that includes animals like jellyfish and coral.
Option 4: Platyhelminthes is incorrect. Platyhelminthes is a phylum that includes flatworms, not crabs.
Therefore, the correct answer is option 3, Arthropoda.