What type of exercise is characterized by long, slow, consistent activity that elevates the horse's heart rate?

The correct choice is aerobic exercise because this type of physical activity involves sustained, long-duration movements that elevate the heart rate and improve the horse's cardiovascular fitness. Aerobic exercise helps to enhance endurance, allowing the horse to perform at a higher level for a longer period of time. During these activities, the horse's body uses oxygen to generate energy efficiently, which is key for maintaining stamina in performance.

In contrast, strength training focuses on building muscle strength and power through resistance exercises, which may not necessarily elevate heart rate consistently over long periods. Warm-up exercises are typically shorter, less intense activities that prepare the horse's body for more strenuous work, while cool-down exercises aim to gradually lower heart rate and prevent stiffness after intense activity. Both warm-up and cool-down exercises serve different purposes than aerobic exercise, which is specifically focused on the improvement of endurance and aerobic capacity.