Which of the following contributes to fuel waste during bus idling?

Leaving the engine running contributes to fuel waste during bus idling because it consumes fuel without providing any productive work or movement. When a bus is idling, the engine continues to burn fuel while not transporting passengers, leading to unnecessary fuel expenditure. In contrast, strategies like efficient route planning or using hybrid technology aim to optimize fuel efficiency and lower emissions, thus reducing waste. Turning off the engine when the bus is not in use is a proactive approach that saves fuel rather than wasting it. Therefore, leaving the engine running is the primary action that directly leads to fuel waste during idling.