答案：C
$\begin{array}{rcl}
y & = & mn^x \\
\log_3 y & = & \log_3 (mn^x) \\
\log_3 y & = & \log_3 (n^x) + \log_3 m \\
\log_3 y & = & x \log_3 n + \log_3 m
\end{array}$
$\begin{array}{rcl}
y & = & mn^x \\
\log_3 y & = & \log_3 (mn^x) \\
\log_3 y & = & \log_3 (n^x) + \log_3 m \\
\log_3 y & = & x \log_3 n + \log_3 m
\end{array}$
留意該直線的斜率 $\log_3 n$。由此，可得
$\begin{array}{rcl}
\log_3 n & = & \dfrac{4-0}{0-(-2)} \\
\log_3 n & = & 2 \\
n & = & 3^2 \\
n & = & 9
\end{array}$
