Question

the efficiency of a machine can be increased by
decreasing distance.
decreasing force.
reducing friction.
reducing work.

Explanation:

Machine efficiency is the ratio of useful output work to total input work. Friction in machines converts input work into wasted heat, reducing useful output. Reducing friction lowers this wasted energy, increasing the proportion of useful work relative to total input work, thus boosting efficiency. Changing distance, force, or reducing work does not directly improve this ratio in a way that increases efficiency.

Answer:

reducing friction.