Final answer:
The centripetal force acting on the runner is friction (option B). Centripetal force is the net external force that acts towards the center of curvature, keeping an object in uniform circular motion.
Step-by-step explanation:
The centripetal force acting on the runner is friction (option B).
Centripetal force is the net external force that acts towards the center of curvature, keeping an object in uniform circular motion.
In this case, as the runner sprints around the circular track, the friction force between the runner's feet and the track surface provides the necessary centripetal force.
Friction acts in the opposite direction to the motion, preventing the runner from sliding outward.
It allows the runner to make the curved path around the track while maintaining a constant speed and without sliding or slipping.