The correct answer is D. 1 hour.
If you went to bed at 9 o'clock and set the alarm for 10 o'clock in the morning, then you would sleep for 9 - 10 = 1 hour.
Option A is incorrect because it would mean that you slept for 13 - 9 = 4 hours before the alarm went off. However, this is not possible because 4 hours is less than the 9 hours between 9 o'clock and 10 o'clock in the morning.
Option B is incorrect because it would mean that you slept for 11 - 9 = 2 hours before the alarm went off. However, this is not possible because 2 hours is less than the 9 hours between 9 o'clock and 10 o'clock in the morning.
Option C is incorrect because it would mean that you slept for 12 - 9 = 3 hours before the alarm went off. However, this is not possible because 3 hours is less than the 9 hours between 9 o'clock and 10 o'clock in the morning.
Therefore, the only possible answer is 1 hour.