A rational number is a number that can be expressed as a fraction of two integers. We can determine whether each of the given square roots is rational by checking if the number inside the square root is a perfect square.
- \( \sqrt{32} \) - This is not a perfect square, so it is irrational.
- \( \sqrt{81} \) - This is \(9\) (since \(9^2 = 81\)), which is a perfect square, so it is rational.
- \( \sqrt{90} \) - This is not a perfect square, so it is irrational.
- \( \sqrt{125} \) - This is not a perfect square, so it is irrational.
Therefore, the rational number among the options is \( \sqrt{81} \).