Final answer:
Fatigue primarily affects a person's decision/reaction time, which is crucial for safe driving and responding to sudden events on the road.
Step-by-step explanation:
What ability does fatigue most likely affect? Fatigue can significantly influence several aspects of bodily functions, primarily impacting vehicle control, vision, and decision/reaction time. When tired, the ability to operate a vehicle safely can be compromised due to decreased reaction times, impaired decision-making, and reduced visual clarity. Consequently, one of the most affected abilities by fatigue is a person's decision/reaction time, which is essential for driving and responding quickly to unforeseen events on the road.