Based on the law of conservation of energy, how can we reasonably improve a machine’s ability to do work?

move the machine to a different gravitational field.

increase the friction between its moving parts.

reduce the friction between its moving parts.

redefine the machine’s system boundaries.

Based on the law of conservation of energy, we know that we can't create energy, machines can only convert one type of energy into another. So, if we want to improve a machines's ability then we need to reduce it's loss energy (part of energy which is useless). Out of all the options only Option C fits best with it.

In short, Your Answer would be Option C

Hope this helps!