The answer is b. False.
The ability to rest one hemisphere of their brains at a time is a phenomenon called unihemispheric sleep. It is not common for the most land mammals. It was observed in some reptiles, birds (for example domestic, chicken), and aquatic mammals (dolphins, seals,...). This phenomenon enabled sleeping animals to receive stimuli all the time. For example, in the case of emergency, animal will receive stimuli to fly away. For aquatic mammals, it is important to emerge to the water surface to breathe while sleeping.
