Which of the following contaminants cannot be effectively removed by ultrafiltration?

Ultrafiltration is a membrane filtration process that is capable of separating particles based on size. It effectively removes pathogens and larger contaminants, such as Giardia and colloids, due to their size, which typically exceeds the pore size of ultrafiltration membranes.

In the context of this question, salts refer to dissolved ions and molecules that are much smaller than the membrane pores of ultrafiltration. Because ultrafiltration operates primarily on the principle of size exclusion, it is unable to retain these small dissolved particles. Therefore, while ultrafiltration can remove larger biological contaminants and particulates, it cannot effectively remove salts, making this the correct answer.