Final answer:
The behavior of nest-building observed in crocodiles and birds was likely also present in their common ancestors, the fossil dinosaurs, making answer A the correct choice.
Step-by-step explanation:
It was hypothesized that since nest-building is an activity shared by birds and crocodiles, fossil dinosaurs also engaged in it. The right response, given the available data, is A. Fossil dinosaurs. The closest surviving cousins of birds are the Crocodilia family of reptiles, which includes alligators and crocodiles.
Through evolutionary history, it is understood that birds are descendants of certain theropod dinosaurs, implying a shared lineage and potentially similar behaviors. Especially since Archaeopteryx, a transition species from dinosaur to bird, shows evidence of feathers and other similar characteristics with birds, and multiple dinosaur species, including theropods, are known to have cared for their young, which supports the behavior of nest-building.