The correct answer is D. Circular shaped.
The other options are incorrect because they are not the most efficient shape for a sewer. A parabolic shape would be more likely to clog, while a horseshoe shape would be more difficult to construct. An egg shape would be somewhere in between, but it is still not as efficient as a circular shape.
A circular shape is the most efficient shape for a sewer because it has the least amount of surface area to volume ratio. This means that there is less friction between the sewage and the walls of the sewer, which allows the sewage to flow more easily. Additionally, a circular shape is the strongest shape for a sewer, which means that it is less likely to collapse.
In olden days, the type of section adopted in trunk and out fall sewers was circular shaped. This is because circular shaped sewers are the most efficient and strongest type of sewer.