1321891

To determine which change in force would stop the box from moving, we need to consider the forces acting on the box and how they affect its motion.

1. **Adding 6 N of force pulling to the right**: This would increase the force to the right and keep the box moving to the right.

2. **Subtracting 8 N of force pulling to the left**: This would decrease the force pulling to the left, allowing the box to potentially continue moving, depending on the remaining forces.

3. **Subtracting 8 N of force pulling to the right**: This would decrease the force pulling to the right, making it less likely to stop the box if there was previously a strong enough force to keep it moving.

4. **Adding 6 N of force pulling to the left**: This would add an opposing force to the motion, potentially stopping or reversing the movement of the box.

To stop the box from moving, the net force acting on it must be zero. **Adding 6 N of force pulling to the left** would counteract any force moving the box to the right, effectively stopping it.

Therefore, the correct response is:

**Adding 6 N of force pulling to the left.**