The correct answer is C) Crankcase oil level. The crankcase oil level should always be checked prior to starting a diesel engine. This is a critical safety check to ensure there is sufficient lubrication in the engine to prevent damage from a lack of oil. Regulations and best practices require engine operators to visually check the oil dipstick and confirm the level is within the proper operating range before attempting to start the engine. The other options, while potentially important checks, are not as critical as verifying the crankcase oil level. Air filters and fuel oil strainers should also be inspected, but issues with them may not immediately cause engine damage upon starting. Pyrometer readings are more relevant during engine operation, not the startup procedure.