The correct answer is "All of these are correct." However, it's worth noting that while mammals are often characterized by traits such as warm-bloodedness and more diverse metabolism, not all mammals are necessarily warm-blooded (e.g., monotremes like the platypus can lay eggs). Additionally, mammals can inhabit a wide range of environments, so the statement about living in only a few types of environments is not accurate. Thus, the most precise answer considering all statements is that "All of these are correct" would not actually be an accurate representation, since some statements are misleading.
The primary characteristics of mammals include having hair or fur, functioning mammary glands for nursing young, and typically being warm-blooded (endothermic).