Radioactive counting rates (number of decay per unit time) follow the Poisson Distribution. (Recall that the Poisson Distribution describes the results of experiments in which one counts events that occur independently, thus at random, but at a definite average rate.) In the PD, , i.e., the standard deviation is approximately the square-root of the average.

Suppose is the average. The square root of that is , hence (A) is the answer.

(Depending on whether one is in the mood for rolling the celestial self-loaded 5-sided dice, one can look for the most obvious relationship between numbers in problems. are two commonly used relationships.)

The Normal (Gaussian) distribution describes the distribution of values for any measurement subject to many sources of error that are all random and small. But, the Poisson Distribution fits this case more closely.