Final answer:
The arrector pili muscle in the dermis contracts in response to cold or fear, pulling on hair follicles and producing goosebumps.
Step-by-step explanation:
The muscle responsible for hair standing up during goosebumps is the arrector pili muscle. This tiny muscle is located in the dermis and is connected to the base of a hair follicle.
When the body is exposed to cold temperatures or experiences fear, the sympathetic nervous system triggers the arrector pili muscles to contract.
This contraction pulls on the hair follicles, making the hair stand up, which is visible on the skin's surface as goosebumps.
This is a reflex that is part of the body's automatic response to help insulate the skin by trapping more air under the hairs.