Ans: C
Sub. $y=0$ into $y=a+\log_b x$, we have
$\begin{array}{rcl}
0 & = & a +\log_b x \\
\log_b x & = & -a \\
x & = & b^{-a}
\end{array}$
Therefore, the coordinates of $S$ are $(b^{-a}, 0)$.
Sub. $y=0$ into $y=\log_c x$, we have
$\begin{array}{rcl}
0 & = & \log_c x \\
x & = & 1
\end{array}$
Therefore, the coordinates of $T$ are $(1,0)$.
$\begin{array}{cl}
& OT : OS \\
= & \dfrac{1 -0}{b^{-a} -0} \\
= & \dfrac{1}{b^{-a}} \\
= & \dfrac{b^a}{1} \\
= & b^a : 1
\end{array}$
