Human Biology: Hormone regulation. Hypothalamus.
The hypothalamus is a portion of the brain that communicates the nervous with the endocrine systems.
Among its function is the regulation of metabolic processes and the autonomic nervous system, such as the regulation of body temperature.
When the body temperature drops, the hypothalamus sends signals to rise it, causing vasoconstriction to avoid heat loss, and shivering, which is the contraction of muscles to produce heat. Another reaction to a low body temperature is the "goose bumps", which erect the body hair to trap the hot air and reduce heat loss.
This means that the correct answer would be C. Muscles and skin.