Dogs have a reduced nonfunctional digit on their paws known as a dewclaw. What is this an example of?

Dogs have reduced non-functional digit on their paws known as dewclaw, this is an example of a vestigial structure. A vestigial structure is an anatomical structure which seems to have no function in the present organism but which was functional in the previous generation of the same organism.

C. Vestigial structure is the correct answer