Final answer:
A. Radula. The protrusible rasping organ in a mollusk's mouth used to scrape food from the substrate is the radula. This organ is equipped with chitinous teeth and varies among mollusk species based on their diet, being absent in bivalves. The radula is distinct from structures like the mandible, proboscis, and buccal mass.
Step-by-step explanation:
The protrusible rasping organ in a mollusk's mouth that is used to scrape food from the substrate is known as the radula. This specialized structure is present in all molluscan groups except for bivalves. The radula operates much like a tongue, equipped with chitinous, file-like teeth, which makes it ideal for shredding or scraping food before it is digested. By moving back and forth, the radula scrapes food particles and directs them into the mollusk's digestive tract. The organ varies among different mollusk species; for example, herbivorous mollusks use it to scrape algae off rocks, while predatory mollusks may use a modified radula to drill into prey's shells. In contrast to the radula, the mandible is a jaw-like structure found in some other invertebrates, the proboscis is a tube-like feeding and sucking organ found in some other animals, and the buccal mass is the mouthpart area containing the radula and associated muscles.
Mollusks also have a distinctive mantle, or pallium, which is the dorsal epidermis that secretes the calcareous shell for protection. However, it is specifically the radula that serves as the feeding apparatus for scraping or grinding food, which is the correct answer to the question given.