1. The problem asks to find the square of a radical expression, but the exact expression is not given. 2. Typically, squaring a radical expression like $\sqrt{a}$ results in $a$ because $\left(\sqrt{a}\right)^2 = a$. 3. Given the options 9, -81, -9, and 81, the likely radical expression is $\sqrt{9}$ or $\sqrt{81}$. 4. If the expression is $\sqrt{9}$, then squaring it gives $\left(\sqrt{9}\right)^2 = 9$. 5. If the expression is $\sqrt{81}$, then squaring it gives $\left(\sqrt{81}\right)^2 = 81$. 6. Since 9 and 81 are positive and squaring a radical always yields a non-negative result, negative options (-9, -81) are invalid. 7. Therefore, the correct answer depends on the original radical, but from the options, 9 or 81 are valid squares. 8. Without the exact radical, the best choice is 9 if the radical was $\sqrt{9}$ or 81 if it was $\sqrt{81}$. Final answer: 9 or 81 depending on the original radical expression.