The sebaceous glands are simple alveolar glands that secrete a substance known as sebum. the secretion of sebum is stimulated the sebaceous glands are simple alveolar glands that secrete a substance known as sebum. the secretion of sebum is stimulated
a. by high temperatures as a protective coating
b. when one is swimming
c. when the air temperature drops
d. by hormones, especially androgens

C. By hormones, especially androgens.

Explanation:

The secretion of sebum is stimulated by hormones. Especially androgens. Sebum can be defined as a complex mixture of lipids synthesized under hormonal stimulation under the sebaceous glands in the skin.  

Androgens can be defined as the sexual masculine hormones and they stimulate the sebaceous secretion docking into specific receptors located in the sebaceous follicle increasing the amount of produced sebum.