Final answer:
Option (c), The reticular activating system (RAS) is the part of the brain that awakens a person from sleep, distinguishing it from the thalamus, which regulates consciousness but doesn't directly cause wakefulness.
Step-by-step explanation:
The part of the brain that functions to wake a person from sleep is the reticular activating system (RAS). It is not the thalamus, limbic system, or primary motor cortex. The RAS is a network of connected nuclei that are located throughout the brainstem.
This system plays a crucial role in controlling arousal and sleep-wake transitions. The thalamus, on the other hand, serves as a gateway for information flowing to and from the cortex, helps regulate consciousness, arousal, and sleep states, but it does not directly wake a person from sleep like the RAS does.