$$y=\sqrt{9-x^{2}} \\ y=\sqrt{9-\left( -x\right)^{2}} \\ y=\sqrt{9-x^{2}}$$

We can narrow down our options by checking for symmetry. We only have to check for $y$-axis symmetry and origin symmetry because none of the options have $x$-axis symmetry. Two of the four options have $y$-axis symmetry, so let's check for that type of symmetry first. To check for $y$-axis symmetry, first replace $x$ with $-x$. Then simplify. The resulting equation is equivalent to the original equation; that means that there IS $y$-axis symmetry.