If two forces act on an object in opposite directions, the force that results is the difference between the two forces.
This is because the forces are acting in opposite directions, and the net force is the force that is left over after the two forces have canceled each other out.
For example, if a force of 10 newtons is acting to the right and a force of 5 newtons is acting to the left, the net force is 5 newtons to the right.
The other options are incorrect. The product of the two forces is the force that would be produced if the two forces were acting in the same direction. The ratio of the two forces is not a force. The sum of the two forces is the force that would be produced if the two forces were acting in the same direction, but it is not the net force if the forces are acting in opposite directions.