ANSWER
[tex]A. \: \frac{xz}{y} [/tex]
EXPLANATION
If quantity x varies directly with y and inversely with z.
The we can write the mathematical statement,
[tex]x\propto \frac{y}{z} [/tex]
We introduce the constant of proportionality k, to obtain,
[tex]x = \frac{ky}{z} [/tex]
We now solve for k, by multiplying through by
[tex] \frac{z}{y} [/tex]
This implies that,
[tex] \frac{xz}{y} = k[/tex]
The correct choice is A.
