Concept: The phylum of an organism is decided by its basic body structure and features.
Explanation:The organism labelled IV is a snail.
Snails are soft-bodied animals that usually carry a spiral shell on their back.
They belong to the phylum Mollusca, which also includes clams, octopuses, and squids.
A typical feature of molluscs is the presence of a muscular foot used for movement.
Because of this soft body and shell, the snail is not a flatworm, not an arthropod, and not a coelenterate.
Therefore, the correct phylum of the organism labelled IV is Mollusca.
Answer: B. mollusca