Of the many impurities commonly found in marine lubricating oil, which of the following CANNOT be removed by a centrifugal purifier at normal operating speeds and temperatures?

Correct Answer: Option C — Diesel fuel oil

A centrifugal purifier relies on density differences between fluids and solids to separate impurities. Because diesel fuel oil and lubricating oil have nearly identical specific gravities and are completely miscible, centrifugal force cannot create the density differential required to separate them.