Final answer:
The fisherman experiences both the tension force from the fishing net and the gravitational force while pulling the net. These forces balance each other when he is stationary due to Newton's second law.
Step-by-step explanation:
The forces acting on the fisherman while pulling the net include two main components: the tension force in the fishing net and the gravitational force acting on the fisherman himself. When the fisherman pulls on the net, according to Newton's third law, there is an equal and opposite reaction force. Therefore, the tension force that the fisherman exerts on the net is matched by a force of equal magnitude exerted by the net on the fisherman, pulling him towards the net. Additionally, the gravitational force acts on the fisherman as it does on all objects with mass, pulling him towards the center of the Earth.
Considering these forces, the net force acting on the fisherman when stationary is zero according to Newton's second law, as the tension force in the net balances the gravitational force exerted on the fisherman's mass. Therefore, the correct answer to the student's question would be 'A. Tension force in the fishing net and the gravitational force.'