The correct answer is D. All the above.
Clogging of sewers can be caused by a variety of factors, including silting, low discharge, and greasy and oily matters.
Silting is the accumulation of sediment in a sewer line. This can happen when water flows through the sewer line at a high velocity, carrying with it small particles of dirt, sand, and other debris. Over time, these particles can build up and create a blockage.
Low discharge is another common cause of sewer blockages. This can happen when there is not enough water flowing through the sewer line to carry away all of the waste. As a result, the waste can build up and create a blockage.
Greasy and oily matters can also cause sewer blockages. This is because these materials are not easily dissolved in water. As a result, they can stick to the walls of the sewer line and create a blockage.
In addition to these factors, sewer blockages can also be caused by foreign objects, such as tree roots, toys, and sanitary products.
If you suspect that your sewer line is blocked, it is important to call a professional plumber to have it inspected and repaired.