Which gland opens into hair follicles and is more active during puberty?

The apocrine gland is associated with hair follicles and becomes significantly more active during puberty. These glands are primarily located in areas such as the armpits and the groin and are responsible for secreting a thicker, milky fluid that can contribute to body odor when broken down by bacteria on the skin's surface.

During puberty, hormonal changes lead to increased activity in apocrine glands, causing them to produce more secretions. This increase in activity is linked to the rise in sex hormones, which influences the development of secondary sexual characteristics, including the maturation of these glands.

In contrast, eccrine glands, which are distributed widely across the body, produce a watery sweat that helps regulate body temperature but do not open into hair follicles. The pineal gland, located in the brain, regulates sleep patterns through the secretion of melatonin and is not involved with hair follicles or sweat production. The adrenal gland produces hormones that affect stress response and metabolism but do not directly involve hair follicles.

Thus, the apocrine gland’s link to hair follicles and its increased activity during puberty makes it the correct choice for this question.