If $\frac{x}{y}=\frac{3}{z}$, then $9 y^{2}$ is equal to:
(a) $\frac{x^{2}}{9}$;
(b) $x^{3} z$;
(c) $3 x^{2}$;
(d) $x^{2} z^{2}$;
(e) $\frac{1}{9} x^{2} z^{2}$.
The correct option is (d).

Since $\frac{x}{y}=\frac{3}{z}$, then $x z=3 y$. Squaring both sides of this equality, we get $x^{2} z^{2}=9 y^{2}$.