To find the net force acting on the runner, you can subtract the air resistance force from the running force:
Net Force = Running Force - Air Resistance Force
Net Force = 325 N (to the right) - 45 N (to the left)
Since the air resistance force is acting in the opposite direction, you can subtract it from the running force:
Net Force = 325 N - 45 N
Net Force = 280 N (to the right)
So, the correct response is:
280 N.