**Explanation:**

The correct answer to the question is option C: Involuntary.

Arrector pili muscles are a type of smooth muscle found in the skin of mammals. These muscles are responsible for causing hair to stand up or "goosebumps" to appear on the skin. They are attached to the hair follicles and contract in response to certain stimuli, such as cold temperatures or emotional states.

**Involuntary Muscles:**

Involuntary muscles are muscles that are not under conscious control. They are controlled by the autonomic nervous system, which regulates bodily functions that are not consciously directed, such as digestion, heart rate, and breathing. The arrector pili muscles fall into this category because their contraction is not something that we can consciously control.

**Smooth Muscles:**

Arrector pili muscles are a type of smooth muscle. Smooth muscles are non-striated muscles that are found in the walls of hollow organs, blood vessels, and other structures throughout the body. They are called "smooth" because they lack the striations (stripes) that are characteristic of skeletal muscles. Smooth muscles are involuntary, meaning that they contract without conscious effort.

**Function of Arrector Pili Muscles:**

The main function of the arrector pili muscles is to contract and cause the hair follicles to stand up. When these muscles contract, they create a slight dimpling of the skin, which results in the appearance of goosebumps. This physiological response is thought to be a vestige of our evolutionary past when our ancestors had more hair and needed to puff up their fur to keep warm or intimidate predators.

**Conclusion:**

In conclusion, arrector pili muscles are smooth muscles that are involuntary, meaning they are not under conscious control. Their primary function is to cause hair to stand up or goosebumps to appear on the skin.

The arrector pili muscles are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end. Each arrector pili is composed of a bundle of smooth muscle fibres, which is attached to several follicles (a follicular unit) and is innervated by the sympathetic branch of the autonomic nervous system. The contraction of the muscle is, therefore, involuntary and stresses such as cold, fear etc., may stimulate the sympathetic nervous system and thus, cause contraction
Thus the correct option is C