Answer is C
Easiest way is to go with the options.
Set of numbers 12, 18, 21 and 28,
→ as 18 is there, we have to identify the options which are divisible by 9. For that, sum of the digits of the options given should be divisible by 9
→ Second criteria, as 21 and 28 are there, required number should be divisible by 7. Only option C is divisible by 7
The option which satisfies both the conditions is 9828