Which hormone increases in response to stress and helps the body respond to emergencies?

The correct choice is adrenaline (epinephrine), which is a hormone that plays a crucial role in the body's immediate response to stress. When an individual encounters a stressful situation, the adrenal glands quickly secrete adrenaline into the bloodstream. This hormone triggers the "fight-or-flight" response, increasing heart rate, enhancing blood flow to muscles, dilating air passages, and raising blood sugar levels for immediate energy. These physiological changes enable the body to respond rapidly and effectively to emergencies.

In contrast, insulin is primarily involved in regulating blood glucose levels and does not play a direct role in stress response. Cortisol, another hormone associated with stress, is released more slowly than adrenaline and is part of the body's longer-term stress response, helping to manage inflammation and metabolism rather than immediate reactions. Thyroxine, a hormone produced by the thyroid gland, regulates metabolism but does not directly respond to acute stress.

Thus, adrenaline is the key hormone for facilitating quick adjustments in the body during stressful situations, making it the correct choice for this question.