Final answer:
The suprachiasmatic nucleus (SCN) in the hypothalamus plays a principal role in establishing the body's circadian rhythm by synchronizing the body's internal clock with the external light-dark cycle.
Step-by-step explanation:
The suprachiasmatic nucleus (SCN) plays a principal role in establishing the circadian rhythm of the body. This small group of cells is located in the hypothalamus and receives direct input from the eyes through the retina. The function of the SCN is to synchronize the body's internal clock with the external environment, mainly by responding to changes in light and darkness, which indicates the time of day to the body. This synchronization helps to regulate sleep-wake cycles, feeding patterns, hormone release, and other vital physiological processes that follow a daily rhythm.
The correct answer to the question is B) suprachiasmatic nucleus.