Rewrite this imaginary number using \( i \) and then select your answer from the choices. Make sure the answer is fully simplified. \( \sqrt{-81} \)

To rewrite \( \sqrt{-81} \) using \( i \), we first recognize that \( i \) is the imaginary unit defined as \( i = \sqrt{-1} \). Therefore, we can express \( \sqrt{-81} \) as: \[ \sqrt{-81} = \sqrt{81} \cdot \sqrt{-1} = 9i \] So the fully simplified answer is \( 9i \).