Which of the following is a feature of mammals?

Mammals are distinguished by several unique features, one of the most notable being the presence of hair or fur. Hair serves multiple important functions, such as insulation, protection, and sensory perception, and it plays a crucial role in regulating body temperature, which is vital for maintaining homeostasis in a warm-blooded (endothermic) organism. This characteristic is fundamental to the mammalian class and is used to classify and differentiate mammals from other animal groups.

In contrast, the other options presented do not define mammals. Gills are associated with aquatic organisms such as fish and amphibians, while feathers are characteristic of birds. The term "cold-blooded" refers to ectothermic animals, which regulate their body temperature through external environmental conditions, unlike mammals which maintain a constant internal temperature regardless of the environment. Thus, hair or fur is the defining feature that identifies mammals, reinforcing their classification within the animal kingdom.