Answer
Elliptical cylinder (graph b).
Work Step by Step
Since, we have $x^2+2z^2=8$
This can be re-written as:
$\dfrac{x^2}{(2 \sqrt 2)^2}+\dfrac{z^2}{(2)^2}=1$
Thus, the above equation is a graph for an Elliptical cylinder along the y-axis as shown in graph (b).
