The missing number is 1.
The numbers in the sequence are all the numbers from 1 to 9, except for 1.
A. 9 is not the missing number because it is already in the sequence. B. 4 is not the missing number because it is not in the sequence. C. 0 is not the missing number because it is not in the sequence. D. 1 is the missing number because it is not in the sequence, but it is a number from 1 to 9.