a) at least speed
b) threshold speed
c) minimum speed
d) idling speed

Answer: d
Explanation: Idle speed is the rotational speed of an engine when the engine is idling. An engine is said to be idle when the engine is not coupled from its drivetrain and the pedal for throttling is not in a depressed position. Idle speed is usually measured in RPM (revolutions per minute). At this speed, the engine will be generating enough power to run smoothly but it won’t be sufficient to perform any useful work.