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 A — Diesel fuel oil

Diesel fuel oil shares a very close specific gravity with lubricating oil, preventing centrifugal purifiers from separating them at normal operating temperatures. Unlike heavy solids or water, soluble hydrocarbon fuels remain blended within the lube oil matrix.