The correct answer is: C. Less than the rate of water supply.
The rate of flow of sewage is generally assumed to be less than the rate of water supply. This is because not all of the water that is supplied to a building or facility will be used. Some of the water will be lost through leaks, evaporation, and other means. Additionally, some of the water will be used for non-potable purposes, such as watering plants or flushing toilets. As a result, the rate of flow of sewage will always be less than the rate of water supply.
Option A is incorrect because the rate of flow of sewage is not always more than the rate of water supply. As explained above, the rate of flow of sewage will always be less than or equal to the rate of water supply.
Option B is incorrect because the rate of flow of sewage is not always equal to the rate of water supply. As explained above, the rate of flow of sewage will always be less than or equal to the rate of water supply.
Option D is incorrect because the rate of flow of sewage is not always 150 litres per capita. The rate of flow of sewage will vary depending on a number of factors, such as the number of people using the facility, the type of facility, and the climate.