Final answer:
The overload principle is improving fitness by placing extra physical demands on the body, achieved by gradually increasing exercise intensity or duration while avoiding injury through proper progression.
Step-by-step explanation:
The overload principle refers to the concept that in order to improve fitness and strength, one must subject the body to additional physical demands beyond those it is accustomed to. This can be achieved by increasing the intensity, duration, frequency, or type of exercise. The correct answer to the question is C) Improving fitness by placing extra physical demands on the body. When the body undergoes physical stress beyond its current capacity, it responds by increasing muscle fiber endurance and strength to better handle such stress in the future. However, it's important to increase the workload gradually to avoid overuse injuries, tendon damage, or other types of harm associated with excessive physical stress.