Which of the following best describes mammals?

Mammals are characterized by several key features, one of the most defining being their ability to regulate their body temperature, which classifies them as warm-blooded animals. This means that mammals maintain a constant internal body temperature regardless of the external environment. Additionally, a distinctive characteristic of mammals is that females possess mammary glands that produce milk to nourish their young, a process known as breastfeeding. This reproductive strategy is crucial for the development and survival of mammalian offspring.

In contrast, the other options highlight characteristics associated with different animal classifications. Cold-blooded animals, such as reptiles or amphibians, rely on environmental temperatures to regulate their body heat. Breathing through gills is a trait of aquatic organisms like fish, which extract oxygen from water. Finally, laying eggs with hard shells is typical of birds and reptiles, rather than mammals. Therefore, option C accurately captures the unique and defining characteristics of mammals.