Answer
$D= x^2+y^2\leq 25$.
Work Step by Step
Since $f(x,y)=\sqrt{25-x^2-y^2}$ is defined for all $x,y$ satisfying that $ x^2+y^2\leq 25$. Hence the domain of $f$ is the closed disc $D= x^2+y^2\leq 25$.
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