An idiom is a phrase or an expression that has a meaning different from the literal meaning.
Option A: 'For good' is an idiomatic expression that means 'permanently'.
It forms a complete and meaningful sentence here: He has decided to permanently leave India. Hence option A is correct.
Option B: 'For bad' is not an idiomatic expression. It has a literal meaning, and does not form a sentence complete in meaning.
Option C: 'For better' is not a complete idiomatic expression.
'For the better' is an idiomatic expression and in terms of change, it means 'to improve'.
Option D: Similarly, 'for worse', or 'for the worse', means 'to become more unpleasant or difficult'.
These options do not form a complete meaningful sentence.
Hence options B, C and D are incorrect.