When issuing a cruise clearance to an aircraft on an unpublished route, what must you ensure?

When issuing a cruise clearance to an aircraft on an unpublished route, it is crucial to issue the appropriate crossing restriction to ensure terrain clearance. This is because unpublished routes may not follow existing airways or provide a clear understanding of the underlying terrain. Therefore, it's the controller's responsibility to provide guidelines that the aircraft must follow to avoid obstacles, ensuring a safe altitude that considers the height of the terrain below. By giving the correct crossing restrictions, you help to maintain safety during the flight, especially since the route is not charted and may encounter unexpected terrain features.

The other choices may pertain to general procedures in air traffic control but do not directly address the primary concern when dealing with unpublished routes. For instance, informing the pilot of airport locations, while useful, is not as critical as ensuring safe altitude management. Regular weather updates are beneficial for flight safety but do not substitute the significance of terrain clearance when the route is not predefined. Monitoring the aircraft's fuel status is generally a concern but is typically the responsibility of the flight crew rather than the controller in this context.