To calculate the plane's net force considering the tailwind, you would add the two forces together (the speed of the plane and the speed of the tailwind).
In this case, the plane is flying at 70 kilometers per hour, and the tailwind is 5 kilometers per hour, so you would calculate:
Net Speed = Speed of Plane + Speed of Tailwind
Net Speed = 70 km/h + 5 km/h = 75 km/h
However, if you're specifically referring to net force in a physics context, the forces acting on the plane due to thrust and drag would need to be assessed based on their direction and magnitude. But since the question focuses on the effects of speed, the best answer among the given options is add the two forces together.