Which muscle is responsible for 'goose bumps'?

The arrector pili muscle is the correct answer because it is the small, smooth muscle located at the base of hair follicles. When this muscle contracts, it causes the hair to stand upright, which is a physiological response often referred to as "goose bumps." This reaction can occur due to various stimuli, such as cold temperatures or emotional responses, and is part of the body's way of regulating temperature or expressing emotions.

The other muscles listed, like the deltoid, trapezius, and latissimus dorsi, serve different functions in the body, such as facilitating arm movement and stabilizing the shoulder, but they are not involved in the response of creating goosebumps. Thus, the arrector pili muscle specifically plays a crucial role in this involuntary reaction.