Refer to figure.
Learning Objective 010.07.02.12.12: Describe track separation between aircraft using the same navigation aid or method.
Source: ICAO Doc 4444, Chapter 5, 5.4.1.2 Lateral separation criteria and minima, 5.4.1.2.1.2

Note: The feedback for this question is incomplete, so you can expect some differences in the exam, especially to the options. Any further exam feedback, as always, is very useful!

When aircraft are navigating via the same navigational aid and also require lateral separation from each other, they are required to fly certain tracks (often diverging by no less than a particular angle) as well as a distance away from a common nav-aid (to make sure they are not too close together.

For VORs, they must fly diverging tracks of no less than 15° and one aircraft must be at least 15 NM away from the VOR, to provide adequate separation.

For NDBs, which are less accurate, they must fly diverging tracks of no less than 30° and one aircraft must be at least 15 NM away from the NDB, to provide adequate separation.

This does mean that there is no set separation for all nav-aids (and when GNSS is used it is different again), so we must answer that it is "according to the navigational aid in use". Again, this may not be the exact wording in the exam.