The answer is A; 1/2. You add 1 1/2 and 1 1/2 together to get 3. Then you subtract 3 1/2 and 3 to get your answer 1/2. I hope that helps!
All possibilities = 9!
possibilities that harry sits at an end = 8! + 8!
possibilities that harry doesn't sit at an end = 9! - 8! -8!