What hormone is known to stimulate the sympathetic nervous system?

Epinephrine, also known as adrenaline, is the hormone that plays a critical role in stimulating the sympathetic nervous system. When released, especially during stress or physical exertion, it prepares the body for a 'fight or flight' response. This includes increasing heart rate, dilating airways, and mobilizing energy stores, which are essential for immediate physical activity.

Cortisol, while it does play a role in the stress response, primarily functions to regulate metabolism and helps the body respond to prolonged stress rather than acutely stimulating the sympathetic nervous system.

Serotonin is primarily known for its role in mood regulation and does not significantly influence the sympathetic nervous system directly.

Insulin is a hormone that regulates blood glucose levels and promotes the uptake of glucose by cells but does not stimulate the sympathetic nervous system.

Thus, the distinct role of epinephrine in enhancing the body's readiness to respond to stressors makes it the correct answer in relation to sympathetic nervous system stimulation.