When can an altitude be assigned to an aircraft?

The assignment of an altitude to an aircraft is considered appropriate once that aircraft has reported passing through a different altitude. This protocol ensures that air traffic control maintains safe vertical separation between aircraft. When one aircraft descends from its original altitude and communicates this to air traffic control, it provides critical information that allows the controller to assign new altitudes to other aircraft operating in the same airspace, ensuring efficient and safe air traffic management.

The other options present situations that do not guarantee safe altitude assignment. Assigning an altitude after a clearance for takeoff does not account for other aircraft that might already be in the system at the same altitude. Holding time does not inherently provide the necessary information regarding the altitude status of other aircraft, and weather conditions alone do not dictate the ability to assign altitudes, as air traffic control must prioritize safety and separation protocols regardless of weather.