Final answer:
Birds are class Aves animals known for their feathers and the lightweight structure, including hollow bones and a high metabolic rate. Feathers provide insulation and are necessary for flight, while the specialized bone structure aids in efficient movement both on land and in the air.
Step-by-step explanation:
The hallmarks of birds include hollow bones (A) and feathers (C). Therefore, the correct answer is D, which is a combination of both B (clawed feet) and C (feathers), although clawed feet are not as unique to birds as the other options. Birds are a group of endothermic vertebrates constituting the class Aves, characterized by feathers, toothless beaked jaws, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton.
The most distinctive feature, which defines all avian species, is the presence of feathers that are essential for flight and insulation, representing a modified form of reptilian scales. The airflow through their lungs is one-directional, unlike mammals, allowing for a more efficient respiratory process which is crucial considering their high levels of activity and high metabolism. Evolved from dinosaurs, birds have retained some similarities such as certain bone structures.