The correct answer is B) Exhaust pyrometer. The exhaust pyrometer is the best indicator of the load being carried by a diesel engine. This is because the temperature of the engine's exhaust is directly related to the engine's workload. As the engine is subjected to a higher load, the exhaust temperature will increase accordingly. The exhaust pyrometer measures this temperature and provides a direct indication of the engine's load. The other options are not as reliable for monitoring engine load. The jacket water temperature gauge and the lube oil pressure gauge can provide some indication of engine condition, but they do not directly reflect the engine's load. The tachometer measures engine speed, but does not necessarily indicate the engine's load.