Final answer:
Circadian rhythms are controlled by the suprachiasmatic nucleus (SCN) of the hypothalamus, which receives light information through the retina to adjust the body's internal clock to the external environment.
Step-by-step explanation:
Circadian rhythms are controlled by a specific area in the brain, and the correct answer to the question 'Circadian rhythms are controlled by the ________.' is D. suprachiasmatic nucleus (SCN).
The suprachiasmatic nucleus (SCN) is located in the hypothalamus. This small but crucial part of the brain receives direct input from the eyes via retina, allowing it to synchronize the body's internal clock with the external environment. Light-sensitive neurons in the retina send signals to the SCN, which in turn helps regulate our sleep-wake cycles and other circadian processes. The SCN communicates these time-keeping signals to another gland, the pineal gland, which is responsible for the production of melatonin, a hormone that influences sleep.
While other brain structures like the thalamus and hippocampus play roles in various other functions, such as sensation and memory, they are not primarily responsible for the control of circadian rhythms. Hence, options A (hippocampus), B (thalamus), and C (pineal gland) are incorrect when identifying the specific location that controls circadian rhythms.