Answer
(a) the statement is correct.
(b) the statement is not correct.
Work Step by Step
(a) If $\phi=0$, then we get $x=0$, $y=0$ and $z=\rho$. So the point lies on the z-axis and the statement is correct.
(b) If $\phi=0$, then we get $x=0$, $y=0$ and $z=\rho$. So the point does not lie on the xy-plane and the statement is not correct.
