The correct answer is B) combustion by-products removed from the cylinder walls. In a normally operating diesel engine, the main source of lubricating oil contamination in the crankcase is the combustion by-products, such as soot and unburnt fuel, that are scraped off the cylinder walls during the engine's operation. These contaminants then mix with the lubricating oil, leading to its gradual degradation and the need for regular oil changes. The other options are incorrect because: A) air cleaners are used to prevent airborne contaminants from entering the engine, C) water vapor condensation is not a major source of oil contamination, and D) metal particles from wear are a secondary source of contamination compared to combustion by-products.