The body's reaction to challenging or threatening situations.

• A. Hormone
• B. Response
• C. Digestion
• D. Stress

Answer: (D)

Stress is the body's coordinated reaction to challenging or threatening situations. When a stressor is encountered, the nervous and endocrine systems jump into action to restore balance and prepare the organism to cope. The sympathetic branch of the autonomic nervous system triggers the fight‑or‑flight response, releasing adrenaline and noradrenaline to raise heart rate, increase blood flow to muscles, and boost energy availability. At the same time, the HPA axis releases cortisol to sustain energy and modulate other bodily functions. This whole, integrated set of changes constitutes the stress response. A hormone is just one messenger involved in this process, and digestion is not the immediate reaction to threat; a generic term like “response” would be too vague to capture the specific, organism-wide reaction to stressors.