This question is related to the very first lesson to be read for Quant preparation, the divisibility priniciple.
Divisibility rules: - Know them well
Integer is divisible by:
2 - if the integer is Even integer
3 - Sum of digits are divisible by 3
4 - Integer is divisible by 2 twice or Last 2 digits are divisible by 4
5 - Last digit is 0 or 5
6 - Integer is divisbile by 2 AND 3
8 - Integer is divisible by 2 three times
9 - Sum of digits is divisible by 9
10 - Last digit is 0
So now come back to your question, if N is divisible by 9 then the sum of all digits is divisible by 9. Hence "D" is the answer.
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