The correct answer is D. 20.
A nuisance is experienced in diluting water if dilution factor is less than 20. This is because the concentration of contaminants in the water will be too high, and the water will not be safe to drink.
The dilution factor is the ratio of the volume of water to the volume of contaminants. A dilution factor of 20 means that there is 20 times as much water as there is contaminants. If the dilution factor is less than 20, then there is not enough water to dilute the contaminants, and the water will be unsafe to drink.
There are a few things that can be done to reduce the risk of experiencing a nuisance when diluting water. One is to use a higher dilution factor. Another is to use a filtration system to remove contaminants from the water. Finally, it is important to test the water to make sure that it is safe to drink before consuming it.