Final answer:
The C) arrector pili muscles are exclusively innervated by sympathetic fibers and are responsible for causing goose bumps.
Step-by-step explanation:
The structure or organ that is innervated exclusively by sympathetic fibers is the arrector pili muscles. These tiny muscles are responsible for causing goose bumps on the skin. The heart, iris, and gallbladder also receive sympathetic innervation, but they are not exclusively innervated by the sympathetic nervous system; they also receive parasympathetic input. For instance, the sympathetic nervous system increases the heart rate, but the parasympathetic system can slow it down. The iris's ability to cause pupil dilation is modulated by both the sympathetic and parasympathetic systems. The gallbladder is similarly influenced by both systems, with sympathetic innervation affecting the release of bile.