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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