The correct option is
A an ir
'Ir', 'un', 'non', 'er' are suffixes added to words, to form new words and new meanings.
Option A: 'Ir' forms a new word 'irredeemable'. The word 'irredeemable' means 'not redeemable, incapable of being saved or improved'.
The word completes the sentence in meaning: the mistake could not be undone.
Hence, option A is correct.
Option B: 'Unredeemable' does not express any meaning of its own. The word does not exist in standard English language.
Option C: 'Non-redeemable' is not a very commonly used term and does not exist in standard English usage.
Option D: 'Erredeemable' does not express any meaning of its own. The word does not exist in standard English language.
Hence, options B, C and D are wrong.