#### Answer

Option C

#### Work Step by Step

We plug in the given point to find the constant of variation. Since the equation will be for direct variation, we can then plug this in for $a$ in the equation $y=ax$. (Note, the y-intercept is 0). Hence, we find:
$$ 18 = 3a \\ a =6$$
Thus:
$$y=6x$$
Option C is correct.