Final answer:
Without additional context, it's not possible to determine which specific item National Airlines is stocking for its CityHopper regional jets among radar transceivers, spare engines, radar displays, or navigation charts.
Step-by-step explanation:
The kind of item that National Airlines is stocking in its maintenance facility for its CityHopper regional jets is not specified in the provided information. As this information is not provided, one would need additional context to accurately determine whether they are stocking items such as radar transceivers, spare engines, radar displays for the cockpit, or navigation charts. Each of these components plays a different role in the operation and maintenance of aircraft. For example, radar transceivers are crucial for air traffic communication, spare engines are necessary for replacement in case of engine failure, radar displays are essential for pilot navigation, and navigation charts are vital for flight planning.