Answer
See below.
Work Step by Step
a. Domain : The domain is set of all $(x,y)$ when $y -x \geq 0$ or, $y \geq x$.
b. Range: $R: z\geq 0$
c. Level Curves: These are straight lines $y-x=c$ $~~~(c\ge 0)$
d. Boundary point is $\sqrt {y-x} =0$ and $y=x$; a straight line.
e. Closed.
f. Unbounded