To find an irrational number between 9 and 11, let's evaluate each option:
- \( \sqrt{100} = 10 \) (This is a rational number).
- \( \sqrt{99} \) is approximately 9.95, and it is irrational.
- \( \sqrt{10} \) is approximately 3.16, and it is not between 9 and 11.
- \( \sqrt{20} \) is approximately 4.47, and it is also not between 9 and 11.
The only number among the options that is both irrational and between 9 and 11 is \( \sqrt{99} \).