The correct answer is $\boxed{2.0}$.
The ratio of maximum sewage flow to average sewage flow for mains up to 1 m in diameter is 2.0. This means that the maximum flow of sewage is twice the average flow. This is because sewage flows are typically highest in the morning and evening, when people are using the most water. The average flow is lower because it takes into account the times when people are not using water, such as during the night.
The ratio of maximum sewage flow to average sewage flow is important because it is used to design sewage systems. The system must be able to handle the maximum flow of sewage, even though this flow only occurs for a short period of time.
The other options are incorrect because they do not represent the correct ratio of maximum sewage flow to average sewage flow.