The correct answer is B) check the crankcase oil level. Before starting a diesel engine, it is crucial to check the crankcase oil level to ensure there is sufficient lubrication for the engine's moving parts. This is a standard practice and a regulatory requirement for the safe operation of diesel engines, as running the engine with an inadequate oil level can lead to severe damage. The other options are not directly related to the pre-start checks for a diesel engine. Cleaning the air filter (A) and changing the fuel oil strainers (C) are important maintenance tasks, but they are not necessarily required before every engine start. Checking the pyrometer readings (D) is more relevant for monitoring the engine's performance during operation, rather than as a pre-start check.