The correct answer is - C) a keeled sternum.
The birds and the turtles have multiple differences in their anatomy, and of the biggest is that the birds have wings, while the turtles don't. Because of the development of the wings, the birds have keeled sternum. The keeled sternum in the birds is an adaptation that supports the ability for flying. The kelp in the birds runs axially along the midline of the sternum, extending outwards, in a perpendicular manner to the plane of the ribs. The turtles lack this kind of anatomy, so it is a safe bet to state that the fossil is a bird one if it includes a keeled sternum.
