The correct answer is A. Photoreceptors, hypothalamus, pineal gland.

Circadian rhythms are the natural 24-hour cycles that regulate various physiological processes and behaviors, such as the sleep-wake cycle, body temperature, hormone release, and digestion. The regulation of circadian rhythms involves the following components:
Photoreceptors: Light-sensitive cells in the retina of the eyes detect changes in light and darkness, sending signals to the brain about the daily light-dark cycle.
Hypothalamus: The suprachiasmatic nucleus (SCN) in the hypothalamus acts as the central “pacemaker” or internal clock for circadian rhythms. It receives input from photoreceptors and coordinates the circadian rhythms throughout the body.
Pineal gland: This small endocrine gland in the brain produces and releases the hormone melatonin, which plays a crucial role in regulating the sleep-wake cycle and other circadian rhythms.
The other answer choices are incorrect:
B. Medulla, thalamus, pituitary gland: These structures are not directly involved in regulating circadian rhythms.
C. Cerebral cortex, motor cortex, adrenal glands: These structures are not directly involved in regulating circadian rhythms.
D. Prefrontal cortex, occipital lobe, cerebellum: These structures are not directly involved in regulating circadian rhythms.
So, the correct answer is A. Photoreceptors, hypothalamus, and pineal gland, which work together to regulate the body’s internal clock and circadian rhythms.