Final answer:
The class Mammalia is unique in having mammary glands, which are used to produce milk for their offspring. While mammals are also warm-blooded, this trait is shared with birds, making mammary glands the distinguishing feature.
Step-by-step explanation:
The class Mammalia contains the only animals that have mammary glands. Mammary glands are specialized structures present in female mammals that produce milk to nourish their young after birth, a process known as lactation. This is one of the primary characteristics that distinguishes mammals from other vertebrates. Additionally, while some mammals do have amniotic eggs, this trait is not exclusive to mammals as it is shared with other amniotes such as reptiles and birds. Hollow bones are primarily found in birds to aid in flight. Mammals are also warm-blooded (endothermic), but this is a trait shared with birds as well. Therefore, the presence of mammary glands is the most distinct characteristic of mammals, which includes a diverse array of creatures such as whales, bats, and humans.