Mollusca includes which of the following organisms?

Mollusca is a diverse phylum of invertebrates that includes animals characterized by a soft body, often with a hard shell. The group comprises a range of organisms such as snails, clams, squids, and notably, oysters. Oysters fall under the class Bivalvia within the Mollusca phylum, which is defined by their two-part hinged shells.

In contrast, the other options represent entirely different animal groups. Insects are classified under the phylum Arthropoda, which is distinct from Mollusca. Sea stars belong to the phylum Echinodermata, known for their radially symmetrical bodies and water vascular systems. Flatworms are part of the phylum Platyhelminthes, recognized for their flattened bodies and lack of a body cavity. Thus, oysters are the correct and relevant representatives of the Mollusca phylum in this list.