The answer is 1.
The missing number is the first number in the sequence. The sequence is a list of numbers that are each 2 less than the number before it. So, the first number in the sequence is 1, the second number is -1, the third number is -3, and so on.
A. 9 is not the missing number because it is not in the sequence. B. 6 is not the missing number because it is not in the sequence. C. 5 is not the missing number because it is not in the sequence. D. 1 is the missing number because it is the first number in the sequence.