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) $\frac{1}{9} x^{2} z^{2}$
Given $\frac{x}{y}=\frac{3}{z}$, then $x z=3 y$. Squaring both sides of this equality, we obtain $x^{2} z^{2}=9 y^{2}$. The correct option is (d).