Ans: A
$\begin{array}{rcl}
\log \dfrac{a}{10} & = & 2 \log b \\
\log \dfrac{a}{10} & = & \log b^2 \\
\dfrac{a}{10} & = & b^2 \\
a & = & 10b^2
\end{array}$
$\begin{array}{rcl}
\log \dfrac{a}{10} & = & 2 \log b \\
\log \dfrac{a}{10} & = & \log b^2 \\
\dfrac{a}{10} & = & b^2 \\
a & = & 10b^2
\end{array}$
