Final answer:
Mammals have hair or fur, which helps regulate body temperature and protect skin. They do not have exoskeletons, are not single-celled, and are not ectothermic.
Step-by-step explanation:
A characteristic of mammals is hair or fur. Mammals are the only animals that have this feature. Hair or fur helps mammals regulate their body temperature and protect their skin.
Unlike mammals, some animals have an exoskeleton (Option A) instead of an internal skeleton. This external skeleton, like that of insects, provides support and protection for the animal.
Option C, single-celled, refers to organisms made up of only one cell, such as bacteria or amoebas. These organisms are not mammals.
Lastly, Option D, ectothermic, refers to animals that rely on external sources to regulate their body temperature, like reptiles, rather than mammals who can maintain a consistent internal body temperature.
Learn more about Characteristics of mammals