The relevant equation is
$\begin{align*}\mbox{probability of scattering} = n t \sigma\end{align*}$
where $n$ is the number density of targets, $t$ is the thickness of the target, and $\sigma$ is the cross section.
$\begin{align*}\sigma = \frac{\mbox{probability of scattering}}{n t} = \frac{10^{-6}}{10^{20} \;\mathrm{cm}^{-3} \cdot 0.1 \mb...$
Therefore, answer (C) is correct.

Solution to 1986 Problem 42