How often must a VOR operational check be accomplished to operate under IFR?

A VOR operational check must be accomplished every 30 days to ensure that the navigation aid is functioning correctly and providing accurate information necessary for IFR (Instrument Flight Rules) operations. This requirement aligns with regulations that are in place to guarantee the safety and reliability of navigation systems used during flight. Regular checks help to avoid potential navigation errors that can lead to dangerous situations in adverse conditions or complicated airspace environments.

The other options might suggest alternative frequencies for checks, but they do not align with the established guidelines for operational safety under IFR. The correct interval of 30 days is specifically mandated to maintain the high standards required for instrument navigation and to mitigate risks associated with inoperative or erroneous VOR signals.