Question

a ____________________ is the gear connected to the input power source, such as a motor. driving gear driven gear gear train gear ratio idler gears

Explanation:

A driving gear is the one that receives power from an input source like a motor. The driven gear is the one that gets power from the driving gear. A gear train is a set of gears. Gear ratio is a ratio related to gears' properties. Idler gears are gears that are in - between (not the input or final output gears in some cases). Since the question is about the gear connected to the input power source, the term is driving gear.

Answer:

Driving Gear