Which of the following is a characteristic of mammals?

Mammals are distinguished by several key characteristics, one of the most prominent being the presence of hair or fur. This trait is essential for regulating body temperature, providing insulation, and in many cases, aiding in sensory perception. The presence of hair or fur is unique to mammals and is a critical part of their biology, contributing to their adaptability in various environments.

The other choices do not accurately reflect characteristics of mammals. For instance, while some mammals do lay eggs, such as monotremes like the platypus, this feature is not a universal characteristic of all mammals. Feathers are specific to birds, and cold-blooded refers to ectothermic animals, which do not include mammals, as they are warm-blooded (endothermic) and maintain a constant internal body temperature. Thus, the defining characteristic of having hair or fur is what sets mammals apart and makes the correct answer clear.