While underway, a slow-speed diesel engine lube oil sump level slowly begins to decrease. Which of the following should be checked?

Correct Answer: Option A — The lubricating oil cooler for leakage into the cooling system.

Under 46 CFR Subchapter F standards, a decreasing lube oil sump level without external leakage typically indicates internal cross-contamination. A breached lube oil cooler allows oil to leak into the higher-capacity freshwater cooling system, which operates at lower static pressures.