Why is a system’s work output limited?

The correct answer is Energy is lost as heat in each stage, causing the amount of energy available to decrease with each stage. This means that eventually energy is bound to run out. Simplest example is a car for example, as long as it works there's less and less fuel, and eventually the energy runs out, or in this case the fuel.

It's definitely not b or c, it's either a or d but i can't figure out which one sorey