To fly a traditional VOR or NDB approach using GPS, what is necessary?

Using GPS in conjunction with a traditional VOR or NDB approach requires adherence to certain navigation standards. The correct choice indicates that a VOR or ADF must be used as the primary source for navigation during the approach. This is because while GPS provides a highly accurate means of navigation, traditional approaches, such as VOR or NDB, typically rely on conventional navigation aids as the primary reference.

In practice, the regulatory framework surrounding such approaches mandates that pilots utilize these traditional navigation sources for critical phases of flight like approaches. GPS can indeed augment navigation; however, its use solely as the primary means during a traditional VOR or NDB approach without backing from the established ground-based navigation system would not comply with established aviation protocols and safety requirements.

The other choices suggest alternatives that either incorrectly imply GPS can function as the sole navigation source or misrepresent the required equipment. Therefore, using a primary navigation source from traditional aids guarantees adherence to procedural integrity during the flight.