Which of the following factors is least likely to affect the quality of drinking water?

The length of plumbing pipes is least likely to directly affect the quality of drinking water compared to the other factors listed. While the length of plumbing pipes can influence some aspects like water pressure and potentially lead to more exposure time for the water to interact with the pipe material, it does not inherently change the fundamental quality of the water itself, such as its chemical composition or contamination levels.

In contrast, the source of the water is crucial as it determines the initial quality and potential contaminants present. Conductivity is a direct measure of the ability of water to conduct electricity, which is related to the presence of ionic substances and can indicate the overall concentration of dissolved salts, thus impacting quality. Water temperature can also affect chemical reactions and the solubility of various substances in water, which can in turn change the water quality significantly. Therefore, while all factors are important in their own right, the length of plumbing pipes has less direct bearing on the actual quality of drinking water.