Ans: B
$\begin{array}{rcl}
a & = & \log_{12} b \\
& = & \dfrac{\log b}{\log 12} \\
\dfrac{1}{a} & = & \dfrac{\log 12}{\log b} \\
& = & \log_b 12
\end{array}$
$\begin{array}{rcl}
a & = & \log_{12} b \\
& = & \dfrac{\log b}{\log 12} \\
\dfrac{1}{a} & = & \dfrac{\log 12}{\log b} \\
& = & \log_b 12
\end{array}$
