Congratulations! Your answer was correct.
It's E. Suprachiasmatic nucleus.

Explanation: Circadian rhythms are controlled by the suprachiasmatic nucleus of the anterior hypothalamus, and the circadian clock controls many internal rhythms, including the sleep-wake cycle, which it keeps in tune with the day-night pattern of the external environment. Circadian rhythms are genetically determined and persist even in the absence of external time cues such as the day-night cycle.

Muskin PR, ed. Study Guide to Psychiatry: A Companion to the American Psychiatric Publishing Textbook of Psychiatry. Sixth Edition. Arlington, VA: American Psychiatric Association Publishing; 2015: 72, 322. Click here to purchase. Members can purchase at a discount.