Final answer:
A moderately active person who becomes more active would gain the greatest health benefits from changing their level of physical activity.
Step-by-step explanation:
The individual who would gain the greatest health benefits from changing their level of physical activity is a moderately active person who becomes more active. Increased physical activity has been found to lower the risk of various diseases, improve overall health, increase bone density and strength, and assist in weight loss and maintenance. Additionally, engaging in more physical activity can lead to a longer life expectancy and have positive effects on mental health and cognitive function.