Explanation:

He must refrain from immoral conducts.
The proper preposition used after the word ‘refrain’ is ‘from’.
The sentence follows the pattern - “please refrain from your bad habits,” where it is being said to quit bad habits.
The refrain has two verb forms - transitive and intransitive.
The Transitive form is obsolete, but the intransitive form is used.
Hence, option 3 is the correct answer.