Final answer:
Flexibility is best defined as the ability of the muscles and joints to move through their full range of motion. Flexibility exercises help to improve this aspect of physical fitness and include activities such as stretching and yoga.
Step-by-step explanation:
The definition that best describes flexibility is A) the ability to move the muscles and joints through a full range of motion. Flexibility involves the capacity of muscles and joints to move through their complete span, from fully extended to fully contracted positions. Flexibility exercises like stretching, yoga, and tai chi are designed to improve joint flexibility, keep muscles limber, and enhance range of motion, thereby helping to decrease the likelihood of injuries. A muscle's ability to return to its original length when relaxed is due to its elasticity, while its capacity to stretch or extend is known as extensibility.