Answer
Sphere with radius $2$ and centre at the origin.
Work Step by Step
Given: $r^2+z^2=4$
As we know that $r^2=x^2+y^2 \implies r=\sqrt{x^2+y^2}$
Thus, we have
$x^2+y^2+z^2=4$
This can be written as:
$x^2+y^2+z^2=2^2$
Thus, it represents an equation of a sphere with radius $2$ and centre at the origin.
