Correct Option: D. Produce an oily substance Detailed Explanation: The sebaceous glands are specialized glands found in the skin of mammals. Their primary function is to produce and secrete an oily substance known as sebum. Here’s a step-by-step breakdown of why option D is correct:

1. Structure of Sebaceous Glands:
2. Sebaceous glands are typically associated with hair follicles. They are composed of clusters of cells that produce sebum, which is a mixture of lipids (fats), including triglycerides, wax esters, and squalene.
3. Function of Sebum:
4. Moisturization: Sebum plays a crucial role in keeping the skin and hair moisturized. It forms a protective barrier that prevents water loss from the skin, helping to maintain hydration.
5. Protection: The oily nature of sebum helps to protect the skin from external factors such as bacteria and fungi. It has antimicrobial properties that can inhibit the growth of certain pathogens.
6. Thermoregulation: By providing a layer of oil on the skin, sebum can help regulate body temperature by reducing water loss through evaporation.
7. Regulation of Sebum Production:
8. Sebum production is influenced by hormonal changes, particularly androgens (male hormones), which can increase sebum production during puberty. This is why many adolescents experience oily skin and acne.

Why the Other Options Are Incorrect:

• Option A: Produce sweat:
• This option is incorrect because sweat is produced by sweat glands (eccrine and apocrine glands), not sebaceous glands. Sweat glands are responsible for thermoregulation and excretion of waste products through perspiration.
• Option B: Secrete sodium chloride:
• This option is also incorrect. Sodium chloride (salt) is primarily excreted through sweat glands. While sebaceous glands do secrete some lipids, they do not secrete sodium chloride.
• Option C: Secrete water:
• This option is misleading. While sebaceous glands do contribute to skin hydration indirectly through sebum, they do not secrete water. Water is primarily lost through sweat glands and is not a function of sebaceous glands.

Summary of Key Points:

• Sebaceous glands produce sebum, an oily substance that moisturizes and protects the skin.
• Sebum helps prevent water loss and has antimicrobial properties.
• Sweat glands, not sebaceous glands, are responsible for producing sweat and excreting sodium chloride.
• Understanding the distinct functions of different skin glands is crucial for comprehending skin physiology and health.

This thorough understanding of sebaceous glands and their functions is essential for anyone studying mammalian biology, particularly in the context of skin health and disease.