Find the missing number?

The answer is 1.

The missing number is the first number in the sequence. The sequence is a list of numbers that are each one less than the previous number. The first number in the sequence is 1, and the second number is 0. The third number is -1, and so on.

Option A is incorrect because 3 is not the first number in the sequence.

Option B is incorrect because 2 is not the first number in the sequence.

Option C is incorrect because 1 is the first number in the sequence.

Option D is incorrect because 9 is not the first number in the sequence.