Under IFR with a VFR-on-top clearance, what altitude should an airship maintain?

When operating under Instrument Flight Rules (IFR) with a VFR-on-top clearance, the airship is required to maintain a VFR cruising altitude that is appropriate for its magnetic course. This means that the aircraft should choose an altitude that adheres to VFR regulations, which are typically based on the direction of travel: odd altitudes plus 500 feet for magnetic courses between 0° and 179°, and even altitudes plus 500 feet for magnetic courses from 180° to 359°.

This requirement ensures that the airship is in compliance with VFR operational principles while still being under IFR, providing both separation from other IFR traffic and compliance with VFR altitude requirements. The other options do not align with the established procedures for IFR operations combined with VFR-on-top clearances. It’s essential to maintain the proper altitude to ensure safety and compliance with both IFR and VFR rules.