Final answer:
A hot air balloon rising at a constant speed of 5 m/s experiences no net force because the force of gravity and the buoyant force are balanced.
Step-by-step explanation:
When a hot air balloon is rising up with a constant speed of 5 m/s, it means that there is no net acceleration. According to Newton's first law of motion, an object moving at constant velocity is either at rest or moving with constant speed in a straight line, unless acted upon by a net force. In the case of the balloon, the forces acting on it must be balanced. Therefore, the force of gravity pulling the balloon down is exactly balanced by the buoyant force pushing it up, resulting in no net force acting on the balloon. The correct answer is A) No net force is acting.