Chapter 7 - Functions and Graphs - 7.5 Formulas, Applications, and Variation - 7.5 Exercise Set - Page 485: 6

$c$

RECALL: When $y$ varies jointly as $x$ and $z$ the variation is represented by the equation $y=kxz$ where $k$ is the constant of variation (constant ratio). Thus, the given equation represents joint variation where $y$ varies directly as $x$ and $z$. Therefore, the answer is $c$.