Solution

The elephant tusk is defined under the genera dentalium. It is a species of scaphopod mollusc. They are greenish fading to white at the tip. This tusk shell is used for burrowing the sand with the help of the boat-like foot. They lack a head, gills and eyes. The radula is simple and the excretion is done by nephridia. These molluscs live on seafloor sediment, feeding on microscopic organisms detritus etc.
Thus, the correct answer is option A.