Final answer:
Epinephrine, released by the adrenal medulla, triggers the fight-or-flight response by preparing the body for rapid action during acute stress.
Step-by-step explanation:
The hormone that is primarily responsible for the fight-or-flight response is epinephrine, also known as adrenaline. This hormone is released by the adrenal medulla during times of acute stress. Epinephrine increases heart rate, and force of contraction, and dilates blood vessels in muscles while constricting blood vessels to non-essential organs, which helps prepare the body for rapid action.