Ans: D
$\begin{array}{rcl}
y & = & ab^x \\
\log_9 y & = & \log_9 (ab^x) \\
\log_9 y & = & \log_9 a + \log_9 b^x \\
\log_9 y & = & \log_9 a + x \log_9 b \\
\end{array}$
$\begin{array}{rcl}
y & = & ab^x \\
\log_9 y & = & \log_9 (ab^x) \\
\log_9 y & = & \log_9 a + \log_9 b^x \\
\log_9 y & = & \log_9 a + x \log_9 b \\
\end{array}$
Consider the slope of the given figure and the above equation, we have
$\begin{array}{rcl}
\log_9 b & = & \dfrac{0-(-2)}{4-0} \\
\log_9 b & = & \dfrac{1}{2} \\
b & = & 9^\frac{1}{2} \\
b & = & 3
\end{array}$
