Final answer:
Birds belong to the class Aves, distinguishing them from other vertebrates and reflecting their qualities such as endothermy, feathers, and flight. Thus option A is the correct answer.
Step-by-step explanation:
Birds are classified into a single class of vertebrates known as Aves. This class separates them distinctly from other vertebrate classes such as Mammalia and Reptilia, reflecting their unique adaptations like endothermy (warm-bloodedness), the presence of feathers, and their ability to fly. Despite birds sharing a common ancestor with theropod dinosaurs, which makes them a part of the broader archosaur clade within the diapsids, the specific class that all modern birds fall under is Aves.