(c) 2
For coincident lines: $\dfrac{a_1}{a_2} = \dfrac{b_1}{b_2} = \dfrac{c_1}{c_2}$
Here, $\dfrac{3}{6} = \dfrac{-1}{-r} = \dfrac{8}{16}$ $\Rightarrow \dfrac{1}{2} = \dfrac{1}{r}$ $\Rightarrow r = 2$
Source: Chapter 3, Section 3.2
For coincident lines, all three ratios of coefficients must be equal. Students often make a sign error with the $b$ coefficients — note that $b_1 = -1$ and $b_2 = -r$, so $\frac{-1}{-r} = \frac{1}{r}$. Setting this equal to $\frac{1}{2}$ gives $r = 2$. Verify: $\frac{3}{6} = \frac{-1}{-2} = \frac{8}{16} = \frac{1}{2}$ ✓