What kind of source is most likely to be correct? unusual source. ancient source. reliable source. relevant source

Reliable.

FOX is relevant, but lies a lot.

Blood letting can help, but ends with death most of the time (ancient)

And an unusual source can be correct, but a reliable source is more often so.

The reliable source is most likely to be correct.Unusual sources are straight up weird, of course not to be trusted until experimented with. Ancient sources are out of date.

i.e. Ancient source: There are 9 planets. Correct There are 8 planets.

A 'relevant' source is not always correct. Just because it is up to date does NOT mean it is correct.

Therefore, your answer is "reliable source".