The correct answer is: B. Twice the average demand.
The design flow of a sewer is the maximum flow that the sewer must be able to carry without overflowing. This flow is typically based on the average demand plus a safety factor. The safety factor is used to account for variations in demand, such as those caused by storms or other events.
A typical safety factor for sewer design is 2. This means that the design flow is twice the average demand. This ensures that the sewer can handle even the most extreme demands.
Option A is incorrect because the average demand is not enough to ensure that the sewer can handle all possible demands. Option C is incorrect because three times the average demand is too much for most sewers. Option D is incorrect because four times the average demand is excessive and would result in a very large and expensive sewer.