Which of the following structures is NOT part of the diencephalon?

The cerebellum is not part of the diencephalon. The diencephalon is a region located above the brainstem, and it includes structures that play critical roles in sensory and motor signal relay, as well as the regulation of functions such as sleep, thirst, and body temperature. The thalamus acts as a major relay station for sensory information, the hypothalamus controls many autonomic functions and regulates hormonal activities, and the subthalamus is involved in regulating movement.

In contrast, the cerebellum is located posterior to the brainstem and is primarily responsible for coordinating voluntary muscle movements, maintaining posture, and regulating balance. Its distinct anatomical location and functions clearly separate it from the diencephalon, thereby making it the correct answer for this question.