If a machine has 100% efficiency, it means that

Well, a device that is 100% efficient, would need to accomplish these things: first: it would need no energy to work second: it wouldn't produce any waste third: it would lose no energy while working fourth: it would work forever fifth: it wouldn't need another force (a human, by example) to operate it. according to this, the only thing i think can be called 100% efficient is a natural magnet. it doesn't consume energy (it even produces it) it doesn't produce any waste, and so on. there you go, i'd say a magnet is perfect to use!

Iwould say a is accurate