The correct answer is A) Oil level. Before starting any diesel or gasoline engine, it is critical to check the oil level to ensure the engine has sufficient lubrication. Running an engine with low oil can lead to severe damage or failure. This is a fundamental requirement outlined in engine operating procedures and safety regulations for marine vessels, including those covered in the US Coast Guard Captain's License Examinations. The other options, while important checks, are not required before starting the engine. Exhaust discharge and cooling water flow are more relevant during engine operation to monitor proper functioning, but do not need to be verified prior to the initial engine start.