All human beings ought to have a moral centre to fall back __________ in times of crisis.

The phrasal verb 'fall back on' means to resort to or rely upon something when other resources fail. The idiom 'fall back on (something) in times of crisis' is standard English usage. 'Fall back into' suggests reversion, 'fall back over' is not idiomatic, and 'fall back along' is incorrect. Therefore, 'on' is the correct preposition completing the phrasal verb 'fall back on'.