Which muscle is responsible for pulling hair straight and causing goosebumps?

The arrector pili muscle is a small, smooth muscle located at the base of hair follicles in the dermis layer of the skin. Its primary function is to contract in response to cold temperatures or emotional stimuli, which pulls the hair follicle into an upright position. This contraction of the arrector pili muscle is what causes the hair to stand up, commonly referred to as "goosebumps."

This physiological response can also serve a protective function in animals, making them appear larger to predators or providing insulation by trapping air when the hair stands erect. The arrector pili muscle plays a significant role in the integumentary system, closely linked to the sensations experienced by the hair follicles.

The other options, while related to the broader context of muscle and skin function, do not specifically identify the muscle responsible for the action described in the question. Smooth muscle refers to a type of muscle tissue but does not specify its role in hair movement. The hair root plexus is a network of sensory nerves that detects hair movement but does not contract or pull hair. Dermal muscle is not a specific anatomical term recognized in the structure of the integumentary system.