The correct option is A 10
Arrange the numbers in the ascending order.
6 7 8 8 9 10 11 12
Observe that the first three numbers 6, 7 and 8 are consecutive natural numbers. The next three numbers 8, 9, and 10 are also consecutive natural numbers. Further, 8 has appeared in both these sets of three numbers. By following the same pattern, the next three numbers should be 10, 11, 12. so, the missing number is 10.