What do we call the network of big and small pipes carrying wastewater?

Sewerage

The correct answer is option 3. Sewerage.

Out of the options you provided, the network of big and small pipes carrying wastewater is called sewerage.

Here's a breakdown of the terms:

Sewer: This refers to an individual pipe that is part of the network and carries wastewater.

Sewage: This refers to the wastewater itself that flows through the sewers.

Sewerage: This term encompasses the entire system of sewers, including the pipes, pumping stations, treatment plants, and other infrastructure involved in collecting, transporting, and treating wastewater.

Therefore, "sewerage" is the most comprehensive term that describes the entire network of pipes used for transporting wastewater.