What must occur before clearance to start the engines is given?

Before clearance to start the engines is granted, it is essential that the post deice check is completed. This process ensures that the aircraft is free from ice and snow, which is crucial for safe engine operation and overall flight safety. The deicing procedure is vital, especially in cold weather conditions, as ice and snow can significantly affect an aircraft's performance and handling characteristics.

The other activities might be important in the overall pre-flight checklist, but they do not directly pertain to engine start clearance. For instance, while the arrival of the ground crew is essential for other operational tasks, it does not specifically relate to engine start authorization. Similarly, while completion of passenger boarding and verification of fuel quantity are necessary for a successful flight, they do not override the critical safety checks associated with ensuring the aircraft is de-iced and ready for engine start.