Which class of animals is known for having hair and mammary glands for nourishing their young?

The class of animals recognized for having hair and mammary glands is Mammalia. Mammals possess hair, which serves various purposes including insulation, protection, and sensory functions. Mammary glands, unique to this class, are crucial for nourishing their young with milk, providing essential nutrients for growth and development. This reproductive adaptation is particularly significant for the survival of offspring.

In contrast, the other classes mentioned do not share these specific characteristics. Reptilia (reptiles) are typically covered in scales rather than hair, and they do not have mammary glands. Amphibia (amphibians) also lack these features and are known for their dual life stages, usually beginning in water with gills and transitioning to a terrestrial phase with lungs. Aves (birds) are characterized by feathers and lay eggs rather than producing milk to feed their young. Thus, the defining features of mammals set them apart from these other classes, making it clear why Mammalia is the correct answer.