Final answer:
The correct sequence of events by which melatonin is secreted in the blood is Exposure to light, pineal gland activation, melatonin release.
Step-by-step explanation:
The correct sequence of events by which melatonin is secreted in the blood is Option B) Exposure to light, pineal gland activation, melatonin release.
When photons of light stimulate the retinas of the eyes, a nerve impulse is sent to a region of the hypothalamus called the suprachiasmatic nucleus (SCN). From the SCN, the nerve signal is carried to the pineal gland via the autonomic nervous system, where the production of melatonin is inhibited. As a result, blood levels of melatonin fall, promoting wakefulness. In contrast, as light levels decline, melatonin production increases, boosting blood levels and causing drowsiness.