The correct answer is $\boxed{\frac{1}{3}}$.
The minimum hourly flow is the lowest flow that is expected to occur in a given hour. The average flow is the total flow divided by the number of hours in the period. The ratio of the minimum hourly flow to the average flow is a measure of the variability of the flow. A higher ratio indicates that the flow is more variable, while a lower ratio indicates that the flow is more consistent.
Option A is incorrect because it is too low. The minimum hourly flow is never less than zero, so the ratio of the minimum hourly flow to the average flow must be at least $\frac{1}{3}$.
Option B is incorrect because it is too high. The minimum hourly flow is never more than the average flow, so the ratio of the minimum hourly flow to the average flow must be less than or equal to $\frac{1}{2}$.
Option C is incorrect because it is not a possible value for the ratio of the minimum hourly flow to the average flow. The minimum hourly flow is never more than the average flow, so the ratio of the minimum hourly flow to the average flow must be less than or equal to $\frac{1}{2}$.
Option D is incorrect because it is too high. The minimum hourly flow is never more than the average flow, so the ratio of the minimum hourly flow to the average flow must be less than or equal to $\frac{1}{2}$.