5
7
8
11
Answer is Right!
Answer is Wrong!
The answer is $\boxed{7}$.
The sequence is a simple alternating addition and subtraction of 1. The first number is 8, and then 1 is added to get 9. Then 1 is subtracted to get 8, and then 1 is added to get 9. This pattern continues, with 1 being added and subtracted alternately. The missing number is therefore the number that comes after 10, which is 7.
Option A, 5, is incorrect because 5 is not in the sequence.
Option B, 7, is correct because 7 is the number that comes after 10 in the sequence.
Option C, 8, is incorrect because 8 is already in the sequence.
Option D, 11, is incorrect because 11 is already in the sequence.