ANS:D - all of these.

1. Inverted Siphon: An inverted siphon is a type of sewer or pipeline arrangement used to convey wastewater or other fluids beneath an obstacle, such as a river, valley, or another utility line, where the natural hydraulic gradient prevents the flow of the sewage. In this arrangement, the pipe is laid below the hydraulic gradient line, which means that the pipe is positioned in such a way that it dips down and then rises back up to the original level after passing under the obstacle. This allows gravity to carry the sewage through the pipe, similar to how a traditional siphon operates, but with the flow direction reversed. Inverted siphons are commonly used in sewerage systems to ensure continuous flow despite natural barriers.
2. Depressed Sewer: The term "depressed sewer" is not a widely recognized term in the field of civil engineering or sewerage systems. It could possibly refer to a sewer line that is laid below the ground level or a sewer system designed to collect wastewater from lower-lying areas. However, without further context or specification, it's difficult to provide a precise explanation.
3. Sag Pipe: A sag pipe, also known as a sag curve or sag bend, is a type of pipeline configuration used to accommodate changes in elevation along the pipeline route. It involves a slight downward curve in the pipeline, creating a low point or "sag" in the pipe. This design allows for the collection of sediment or debris at the low point, which can be cleaned or flushed out periodically. While sag pipes are not typically used specifically for passing sewer lines beneath obstacles, they are relevant in pipeline design for managing changes in elevation and maintaining flow efficiency.