To determine which of the numbers is a rational number, we need to check whether each of the square roots is a whole number (an integer) since square roots that are whole numbers are rational.
- \( \sqrt{32} \approx 5.656 \) (not a rational number)
- \( \sqrt{81} = 9 \) (a rational number)
- \( \sqrt{90} \approx 9.487 \) (not a rational number)
- \( \sqrt{125} \approx 11.180 \) (not a rational number)
Among the numbers given, \( \sqrt{81} \) is a rational number because it equals 9, which is an integer.
Therefore, the answer is \( \sqrt{81} \).