Define the term 'aerobic exercise.'

Aerobic exercise is characterized by activities that elevate the heart rate and increase breathing, leading to improved cardiovascular fitness. This type of exercise relies on the body's ability to use oxygen efficiently during prolonged physical activity. Activities such as running, swimming, cycling, and dancing fall under this category, as they enhance the heart and lung capacity, improving overall stamina.

While the other choices represent important aspects of physical fitness—the building of muscle mass, increasing flexibility, and high-intensity exercises performed in short bursts—none of them accurately define aerobic exercise. Building muscle mass is more related to resistance training, flexibility is improved through stretching routines, and short bursts of activity refer to anaerobic exercise. Thus, the correct answer is centered on the fundamental role of aerobic exercise in enhancing cardiovascular health through sustained activity that requires oxygen consumption.