Final answer:
An idler gear inserted between a driving and a driven gear maintains the original speed ratio between them.
Step-by-step explanation:
An idler gear inserted between a driving and driven gear does not alter the speed ratio between them because it simply transfers the rotational motion from the driving gear to the driven gear without changing the speed or direction of the rotation.
The function of the idler gear is to maintain the original speed ratio established by the sizes of the driving and driven gears. Therefore, the correct answer is c) Maintains speed ratio. For example, consider two gears in direct contact with each other.
Where gear A has a size that allows it to rotate twice for every one rotation of gear B. Introducing an idler gear between them would not change this 2:1 speed ratio since the idler gear's teeth simply pass along the motion.