Which part of the brainstem is primarily involved in controlling arousal?

The reticular formation is the correct answer because it plays a crucial role in regulating arousal and the sleep-wake cycle. This network of neurons is located in the brainstem and extends from the spinal cord up into the thalamus. It is responsible for filtering incoming stimuli and determining which sensory messages are sent to the higher brain centers for further processing, a process that significantly influences attention and alertness. The reticular formation's activation promotes wakefulness and alertness, while its inhibition can lead to drowsiness and sleep.

Other structures listed in the choices have different functions. The hippocampus is primarily involved in memory formation and spatial navigation. The amygdala is integral to processing emotions, particularly fear and pleasure. The hypothalamus plays a key role in maintaining homeostasis, regulating autonomic functions like hunger and thirst, and controlling the endocrine system through hormone release. While these structures are important for various aspects of behavior and physiology, they do not have the primary controlling influence over arousal like the reticular formation does.