To find the net force acting on the runner, you subtract the force of air resistance from the running force.
The runner exerts a force of 325 N to the right, and the air resistance exerts a force of 45 N to the left.
Net Force = Running Force - Air Resistance Force
Net Force = 325 N (to the right) - 45 N (to the left)
Net Force = 325 N - 45 N = 280 N (to the right)
So, the correct response is 280 N.