Answer
$C,\quad II$
Work Step by Step
Interchanging x and y, we get the same function $\rightarrow$ symmetry about the plane x=y.
The function is periodic in x and in y.
When either x or y is 0, the function is 0 (the x and y axes are on the graph).
Graph C is the only one to satisfy these conditions.
C is associated with level curves II.
