The correct answer is A. Circular sewer.
A circular sewer is the most efficient type of sewer because it has the least amount of surface area to volume ratio. This means that it is less likely to clog and is easier to clean. Circular sewers are also the most common type of sewer, so they are more cost-effective to install and maintain.
An egg-shaped sewer is a type of sewer that is wider at the top than it is at the bottom. This shape is designed to reduce the amount of turbulence in the sewer, which can help to prevent clogging. However, egg-shaped sewers are more expensive to install and maintain than circular sewers.
A rectangular sewer is a type of sewer that is wider than it is tall. This shape is designed to maximize the amount of flow that can pass through the sewer. However, rectangular sewers are more likely to clog than circular sewers.
Therefore, the best type of sewer for a combined system of sewage is a circular sewer.