The correct answer is A. 10 litres.
Sludge is a solid or semi-solid waste generated from the treatment of wastewater. It is a mixture of organic and inorganic matter, and can contain a variety of contaminants, including pathogens, heavy metals, and nutrients.
The rate of accumulation of sludge per person per year varies depending on a number of factors, including the type of wastewater treatment plant, the efficiency of the treatment process, and the amount of water used per person. However, a typical value is 10 litres per person per year.
Option B, 15 litres per person per year, is a higher value than the typical rate of accumulation. This could be due to a number of factors, such as a less efficient treatment process or a higher amount of water used per person.
Option C, 20 litres per person per year, is a much higher value than the typical rate of accumulation. This could be due to a number of factors, such as a very inefficient treatment process or a very high amount of water used per person.
Option D, 25 litres per person per year, is an even higher value than Option C. This is an extremely high value for the rate of accumulation of sludge per person per year. It is unlikely that this value would be observed in practice.