<em>The correct option is C) direction of force</em>
Explanation:
In the terms of physics, force can be described as a push or pull on an object which allows the object to either move forward or backwards or in other directions.
When Dr. Jeff uses the fire extinguisher of the skateboard to move forward, he will require the same force to move backward but in the opposite direction. Hence, for moving backwards, the only thing required by Dr. Jeff to move backwards will be the direction of the force.
