Question

small endocrine gland that produces the hormone melatonin, which is involved in regulating sleep - wake cycles
nerve cell
nerve tissue
gi tract
brain stem
pineal gland

Explanation:

The pineal gland is a small endocrine gland that secretes melatonin, which helps regulate sleep - wake cycles. Nerve cells and nerve tissue are related to the nervous system, not melatonin production. The GI tract is involved in digestion. The brain - stem has functions related to basic life - sustaining processes like breathing and heart rate, not melatonin production.

Answer:

E. pineal gland