What type of organism is a mollusk?

Mollusks are indeed often characterized as "shell organisms" because many of them possess a hard external shell made of calcium carbonate. This shell serves a protective function for the soft-bodied animal inside, which is a common trait among various mollusks, including snails, clams, and oysters. The shell can also provide structural support and a point for muscle attachment. While not all mollusks have an external shell, such as octopuses, they are primarily recognized by their distinct body plan, which typically includes a muscular foot, a visceral mass, and a mantle that can secrete the shell.

The other options do not accurately represent mollusks. They are not mammals, plants, or fungi, as they belong to a separate category within the animal kingdom known as invertebrates. Understanding these classifications helps highlight the unique characteristics that define mollusks and distinguish them from other life forms.