The human biological clock is governed by a cluster of cells known as the ________ of the hypothalamus.

A) basal ganglia
B) reticular activating system
C) suprachiasmatic nucleus
D) substantia nigra

The suprachiasmatic nucleus (SCN) is a group of cells located in the hypothalamus that serve as the body's internal clock. It helps regulate our daily sleep-wake cycle, hormone release, and many other physiological processes. The basal ganglia, reticular activating system, and substantia nigra are all important brain structures, but they are not directly involved in regulating the biological clock.