Answer
a. $\mathrm{F}$
b. $\mathrm{I}$
Work Step by Step
When $x=y$ (the plane $x-y=0)$, the function $f(x,y)$ is constant.
It is constant along every plane parallel to it ($x-y-k=0).$
This is graph F, and the level curves associated with it are (I).
