Answer: The correct answer is-
Vestigial structure.
Vestigial structures are the parts/ organs present in a species, which were functional in their ancestors but serve no function in them that is they are not used by the present species.
Dogs possess reduced and nonfunctional digit on their paws ( particularly located on the inner side of the front leg) called as a dewclaw. They serve no function in dogs.
Thus, dewclaw is an example of Vestigial structure