The missing number is 6.
The numbers in the sequence are increasing by 2, except for the last number, which is decreasing by 1. Therefore, the missing number is 6, which is 2 more than the previous number, 5, and 1 less than the last number, 7.
A. 7 is not the missing number because it is not in the sequence.
B. 1 is not the missing number because it is not in the sequence.
C. 8 is not the missing number because it is not in the sequence.
D. 6 is the missing number because it is in the sequence and it is 2 more than the previous number, 5, and 1 less than the last number, 7.