The correct answer is D) piston ring blow-by. A decrease in the time interval between lube filter cartridge replacements on a small diesel engine indicates increased engine wear, specifically piston ring blow-by. When piston rings become worn, they allow combustion gases to leak past the rings and into the crankcase, contaminating the engine oil. This increased blow-by results in the oil becoming dirtier at a faster rate, requiring more frequent filter changes. The other options are incorrect because they do not directly cause increased contamination of the engine oil. Excessive oil temperature or pressure may contribute to engine wear over time, but would not lead to a noticeable decrease in oil filter service life. A dirty air filter may restrict airflow and affect engine performance, but would not directly impact the oil contamination level.