Answer
(a) The statement is not correct.
(b) The statement is correct.
Work Step by Step
In cylinderical coordinates, we have
$$
\begin{aligned}
&x=r \cos \theta\\
&y=r \sin \theta\\
&z=z
\end{aligned}
$$
(a) When $\theta =0$, then $x=r, y=0$ and $z=z$. Hence, the statement is not correct.
(b) When $\theta =0$, then $x=r, y=0$ and $z=z$. So the point lies on the $xz$-plane. Thus, the statement is correct.
