One must learn (1)/ to distinguish (2)/ good from bad. (3)/ No error (4)

Answer

3

Explanation

Def.Art. the is used with Adj. to refer to a thing or a group of people described by the Adj.
Look at the examples given below:
At what age are children able to distinguish between right and wrong?
It was hard to distinguish one twin from the other.
Both sentences are correct.
It is better to use between good and bad Hence the good from the bad is the right usage.