Final answer:
The net force is the sum of all forces on an object. If the dog does not move when Scott pulls with a force of 25N, it means the forces are balanced and the net force is 0N.
Step-by-step explanation:
The net force on an object is the vector sum of all the forces acting on it. If Scott tugs at his dog's leash with 25N of force and the dog does not move, this indicates that the forces are balanced. The force that Scott applies is apparently being countered by an equal and opposite force from the dog's inertia or friction between the dog's feet and the ground, resulting in no movement. According to Newton's first law, if an object does not move, this implies the forces acting on it are in equilibrium, and therefore, the net force is zero.
Therefore, the correct answer to the student's question is b) 0N.