Which navigational instrument uses non-directional radio beacons to navigate?

The correct option is ADF, which stands for Automatic Direction Finder. This instrument relies on non-directional radio beacons (NDBs) to help pilots navigate. NDBs transmit radio signals in all directions, and the ADF in the aircraft detects these signals. By indicating the direction to the NDB, the ADF allows pilots to determine their bearing relative to the beacon, which is useful for navigation, especially in areas without visual references.

In contrast, VOR (VHF Omnidirectional Range) uses directed signals to provide more precise navigation than NDBs, making it a different navigational aid. ILS (Instrument Landing System) is specifically used for landing approaches rather than general navigation, and RNAV (Area Navigation) allows for navigation along defined flight paths using various types of signals, including GPS, rather than relying solely on NDBs. Therefore, the ADF's unique use of non-directional radio beacons directly fits the criteria given in the question.