Final answer:
The regulation of pupil size is a uniquely sympathetic function, as the sympathetic nervous system is responsible for pupil dilation, while other options involve both sympathetic and parasympathetic control.
Step-by-step explanation:
The autonomic nervous system has two main divisions: the sympathetic and parasympathetic nervous systems. Both divisions uniquely contribute to bodily functions, often with opposing effects to maintain homeostasis. The sympathetic division is responsible for the 'fight or flight' response, triggering physiological changes that prepare the body to face a threat or flee from it. Specifically, the sympathetic nervous system is known for functions such as pupil dilation to allow more light into the eye, increasing heart rate, and constricting blood vessels, which contributes to blood pressure regulation. Conversely, the parasympathetic system promotes 'rest and digest' functions, such as decreasing heart rate, constricting pupils, and facilitating digestion.
Given the unique roles of the sympathetic nervous system, the correct answer to the question 'Which is a uniquely sympathetic function?' is (D) regulation of pupil size. This is because pupil dilation is solely controlled by the sympathetic nervous system, while the regulation of body temperature, respiratory rate, and cardiac rate also involve parasympathetic influences.