Final answer:
The mantle and mantle cavity are distinctive features of organisms in the phylum Mollusca. So, correct option is (A).
Step-by-step explanation:
A mantle and mantle cavity are characteristic features of organisms within the phylum Mollusca. The mantle is a significant part of the mollusk's body, it is a flap of tissue that covers the visceral mass, creating the mantle cavity. This structure can secrete a shell of calcium carbonate in many species for protection.
The mantle cavity is notable for housing several important organs and serving various functions. Aquatic mollusks have gills or ctenidia that facilitate respiration within this cavity, while terrestrial mollusks use the mantle cavity itself for gas exchange.
Moreover, the mantle cavity is distinct from the coelomic cavity, which in adult mollusks is largely reduced to surrounding the heart and other internal organs. By contrast, other invertebrate phyla, such as Annelids, Echinoderms, and Arachnids, do not have this mantle structure.
Therefore, the presence of a mantle and mantle cavity is indicative of mollusks, hence the correct answer is phylum Mollusca.