Question

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

Explanation:

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

Answer:

pineal gland