Answer:
A. the floor pushing up on the player
Explanation:
This is because, when the player takes a stride, he puts force into the floor. The floor now has too much mass for the athlete to move it so, it travels back to the athlete and propels him forward. Because the floor will apply an equal and opposite reaction, whichever direction the athlete applies force will be opposite to the direction force is applied back. If the athlete's foot pushes the floor behind him, the force from the floor which is called ground reaction, will propel him forward. If the athlete quickly applies force straight down, the ground reaction will propel him straight up and allow the athlete to jump.