What does the sympathetic nervous system (SNS) regulate?

The sympathetic nervous system (SNS) primarily regulates the body’s involuntary responses to stress and functions in the "fight or flight" response. One of its key roles is to influence immune function in vivo, which includes modulating immune responses in the body during periods of stress. The activation of the SNS can lead to various physiological changes, such as increased heart rate and blood pressure, but it also impacts how the immune system functions, such as enhancing certain immune responses or suppressing others depending on the need.

This intricate regulation of immune function is due to the relationship between the nervous system and the immune system, where neurotransmitters released by the SNS can directly affect immune cells. Therefore, the answer emphasizing the SNS's role in all aspects of immune function aligns best with established knowledge in this area.

Other options do not capture the comprehensive regulatory role of the SNS adequately. For instance, the SNS does impact cognitive functions, emotional responses, and physical reflexes, but it does so as part of a broader system that does not confine its regulation to a single aspect. Instead, it integrates physiological responses across various domains, making the choice regarding immune function the most accurate representation of the SNS's role.