Answer
$D={(x,y,z) | x^2 / 4 + y^2 / 4 + z^2 / 16 < 1}$
Work Step by Step
The argument of a natural log must be greater than zero. So
$16-4x^2 -4y^2 - z^2 >0$.
Then rearrange we get
$x^2 / 4 + y^2 / 4 + z^2 / 16 < 1.$
And this graph of this is an ellipsoid and it is the set of points inside the ellipsoid.
