A linear function models the total cost $y$, in dollars, for a printer and $x$ ink cartridges. The graph passes through $(0, 80)$ and $(1, 95)$. The slope represents which of the following?

The slope is the change in $y$ per unit increase in $x$. From $(0, 80)$ to $(1, 95)$, $y$ increases by 15 as $x$ increases by 1, so slope = 15. In context, $y$ is total cost and $x$ is number of cartridges, so the slope is the cost per cartridge, i.e., $15.