Answer:
The correct answer is option c, that is, keeled sternum.
Step-by-step explanation:
In the given question, there is a need to access the skeleton or the fossilized remains of the species. The turtles possess bony, tough shells that safeguard their body. It exhibits a domed top (carapace) and a flat layer underneath the belly (plastron). The backbones of the turtles are amalgamated with the bones in their shells.
While, the birds exhibit a lightweight skeleton, which is formed majorly of thin and hollow bones. If the skeleton possesses a breastbone, or the keeled sternum, then what they have found is most probably the nest of the bird.