When may an operator allow an engine of a vehicle to remain in operation while stationary?

Allowing the engine of a vehicle to remain in operation while stationary is permitted in specific circumstances where it is necessary for the vehicle's function or operational efficiency. The operation of ready mix trucks or cranes requires the engines to run while stationary because they need to maintain power for their operations. For instance, ready mix trucks must keep the drum spinning to prevent the concrete from hardening, while cranes often need to ensure hydraulic systems are operational for load handling.

In contrast, fueling operations typically require engines to be turned off to minimize risks associated with fueling around flammable materials. Similarly, waiting for passenger pick-up does not necessitate keeping an engine running, as there are alternatives like using auxiliary power. Routine maintenance generally necessitates the engine being off to ensure safety and compliance with protocols. Thus, the scenarios where the engine may be operational while stationary are limited to those that directly relate to operational necessity, making the involvement of ready mix trucks or cranes the correct answer.