Final answer:
Simple machines can increase or decrease force, which is a function of their mechanical advantage; they do not generate energy but modify force requirements while conserving energy.
Step-by-step explanation:
Simple machines are fundamental devices that modify force and distance to make work easier. The correct answer to the question, 'Simple machines can ____,' is C. Increase or decrease force. This property of simple machines is due to their ability to change the amount of input force required, often by compensating through a change in the distance over which the force is applied.
The concept of mechanical advantage (MA) is crucial here, which is the ratio of output force to input force magnitudes for any simple machine. Examples of simple machines include levers, pulleys, and screws, all designed to conserve energy while altering force.