Answer:
D. a vestigal structure
Step-by-step explanation:
The type of evidence for common descent that would include the whale's pelvic girdle is vestigeal structure.
Vestigeal structures are small, degenerated and non-functional organs found in organisms which are believed to have been well developed and functional in their ancestors.
The pelvic girdle in whales are considered to be vestigeal organs because they serve no known purpose and looks degenerated. Hence, they serve as evidence for common descent of mammals from the same ancestors.
The correct option is D.