Explanation:

8439x53 will be divisible by 99 if it is divisible by both 9 and 11.
Rule for 9:A number is divisible by 9 if the sum of the digits is divisible by 9
Rule for 11:Subtract and then add the digits in an alternating pattern from left to right. If the answer is 0 or 11, then the result is divisible
by 11
If x= 4 , then sum would be 36, satisfying the rule for 9.
Now , check for 11 : 8 - 4 + 3 - 9 + 4 - 5 + 3 = 0
So , the answer would be option b)4.