Answer
Elliptical (circular) cone.
See image:
.
Work Step by Step
Dividing with 4, we rewrite the equation as:
$\displaystyle \frac{x^{2}}{1^{2}}+\frac{y^{2}}{1^{2}}=\frac{z^{2}}{2^{2}}$
and comparing to Table 11.1, we recognize the form of
Elliptical cone.
The cross sections with planes $z=k$ are circles
(at $z=\pm 2,$ the radius is 1).
We know that circles are special cases of ellipses.
