**Core Concept**
Animal hair is a type of keratinized filamentous structure that grows from hair follicles in mammals, serving as a key component of the integumentary system. It is composed of a tough protein called keratin, which provides strength and rigidity to the hair shaft.

**Why the Correct Answer is Right**
The correct answer highlights a fundamental characteristic of animal hair. Hair growth is influenced by hormones, particularly androgens, which stimulate the growth of hair in the anagen phase. The hair shaft is composed of three layers: the medulla, the cortex, and the cuticle. The cortex is the thickest layer and contains keratin filaments that provide strength and elasticity to the hair.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because animal hair does not have a significant role in thermoregulation, unlike fur or feathers in other animals.
**Option B:** This option is incorrect because while it is true that hair can be pigmented, this is not a defining characteristic of animal hair.
**Option C:** This option is incorrect because the sebaceous glands are associated with the production of sebum, not hair growth.

**Clinical Pearl / High-Yield Fact**
It is essential to note that abnormal hair growth patterns can be indicative of underlying endocrine disorders, such as hyperthyroidism or hypothyroidism, which can cause changes in hair texture, growth rate, or distribution.

**Correct Answer: D. A hard protein called keratin.**