Final answer:
The nervous system is the organ system capable of producing the most focused and quickest responses, exemplified by the fight-or-flight response.
Step-by-step explanation:
The organ system known for producing more focused, direct, and the fastest responses from target cells is the nervous system. This system is responsible for quick reactions to changes in the external environment, such as the fight-or-flight response, where signals from the brain cause the secretion of hormones like adrenaline and cortisol. The nervous system can swiftly influence other organ systems and cause rapid responses in them, such as increasing the heart rate, dilating the pupils, and redirecting blood flow to essential organs. The autonomic nervous system, especially the sympathetic division, plays a key role in stimulating various effector organs during a threat to coordinate a systemic change, activating the cardiovascular, respiratory, and musculoskeletal systems simultaneously.