Which of the following is an indicator of a strong cardiovascular system in athletes?

A low resting heart rate is an indicator of a strong cardiovascular system in athletes. This physiological response reflects the efficiency of the heart; a lower resting heart rate signifies that the heart can pump a sufficient amount of blood with fewer beats. This efficiency is a result of cardiovascular conditioning, where regular aerobic exercises improve the heart's ability to deliver oxygen to the muscles and remove carbon dioxide and other waste products from the bloodstream.

When athletes engage in consistent endurance training, their hearts adapt by becoming stronger and more efficient, often leading to a lower heart rate at rest. Additionally, a lower resting heart rate is generally associated with better overall cardiovascular health, as it indicates the heart is working less strenuously when a person is at rest.

In contrast, indicators such as high cholesterol levels, frequent muscle cramps, and a high body fat percentage generally suggest potential health issues or inferior fitness levels, rather than strength in the cardiovascular system.