The correct option is A ment
When we add the suffix 'ment' to the word 'better', we get the word 'betterment'. We can use it in a sentence like... 'Rohit's parents got him admitted in a good private hospital for the betterment of his health.' Thus, Option A is correct.
The other options are incorrect because the suffixes 'ful', 'ing' and 'ary' don't justifiably go with the word 'better'.