A radula is a unique organ for feeding found in:
A. molluscs
B. annelids
C. arthropods
D. All of the above

Hint:Radula are minutely toothed anatomical structures used for feeding. They are generally used for scraping or cutting food before the food enters the pharynx. It mainly consists of the radular membrane, hyaline shield, odontophore (flesh under the tongue) and the teeth. It is mostly used for grazing algae and diatoms (microalgae) off the rocky surfaces.

Complete answer:
 >It has been already stated that the radula is mostly used for grazing activities.
>We know that grazing is mostly associated with herbivores.
>The annelids are generally carnivorous. They possess powerful and sharp jaws in the posterior end of their pharynx. They can protrude out their pharynx by protractor muscles for feeding on their prey and can withdraw it back by retractor muscles. Thus, this option is also not correct.
>Arthropods are animals exhibiting every type of feeding mode. They can be carnivores, herbivores, detritus feeders, filter feeders, and parasites. But they don’t possess radula. So, this option is also not correct.
>Molluscs graze on algae. Thus, they are herbivorous. They possess the radula for feeding activities.

Note:We know that the molluscs are the largest marine phylum and have about eighty-five thousand living species. They mostly occur in freshwater and on land. Around 23% of their species have been found in water bodies. All molluscs share some unique characteristics that define their body plan. In general, their bodies have a head, a foot and a visceral mass.