Final answer:
Yoga is the best option for promoting flexibility because it involves stretching and lengthening muscles, which enhances the range of motion and reduces injury risk, unlike weightlifting, sprinting, or cycling.
Step-by-step explanation:
The best activity that promotes flexibility among the options provided is yoga. Flexibility exercises are any physical activities that stretch and lengthen muscles, enhancing the range of motion and reducing the risk of injuries. While aerobic exercises like cycling promote cardiovascular health and muscular endurance, and anaerobic exercises like weightlifting and sprinting increase muscle strength, it is yoga, with its various stretches and postures, that is particularly effective at improving flexibility.