The correct answer is C) According to manufacturer's instructions. This is the correct answer because the frequency of changing the lubricating oil in a diesel engine is typically determined by the engine manufacturer based on their specific recommendations and requirements. The manufacturer takes into account factors such as engine type, operating conditions, and usage patterns to specify the optimal oil change interval to ensure proper lubrication and engine performance. The other answer choices are not correct because: A) changing the oil after every trip may be excessive and unnecessary; B) changing the oil every 4000 hours may not be appropriate for all diesel engines; and D) changing the oil every time the engine is shut down is not a standard practice and could be overly frequent.