The counts are distributed according to the Poisson distribution. The mean $\mu$ and standard deviation $\sigma$ of a Poisson distribution are related by
$\begin{align*}\sigma^2 = \mu\end{align*}$
We take $\mu \approx 10000$ counts/hour which gives $\sigma \approx 100$ counts/hour. Hence, answer (A) is correct.

Solution to 1986 Problem 40