What type of gland secretes a thick, sticky fluid that includes cellular waste and fatty substances?

The apocrine gland is the correct choice because it is responsible for secreting a thick, sticky fluid that contains cellular waste and fatty substances. These glands are primarily found in areas with many hair follicles, such as the underarms and groin, and they become active during puberty. The secretions from apocrine glands are associated with body odor, as they are broken down by bacteria on the skin's surface.

In contrast, eccrine glands produce a more watery sweat that helps regulate body temperature and does not contain fatty substances, but instead consists mainly of water and salt. The thyroid gland plays a role in regulating metabolism and does not secrete sweat or other skin-related substances, while the adrenal gland produces hormones like adrenaline and cortisol as part of the body's response to stress rather than any type of fluids related to the skin.