The correct answer is C) Exhaust temperature. A pyrometer is a device used to measure the temperature of exhaust gases in a diesel engine. It is essential for monitoring the performance and efficiency of the engine by tracking the exhaust temperature, which can indicate issues such as fuel injection problems, turbocharger malfunctions, or excessive engine load. The exhaust temperature is a critical parameter for maintaining the proper operation and safety of a diesel engine. The other options are incorrect because water pressure (A) and air box pressure (D) are not directly measured by a pyrometer, and water temperature (B) is typically measured by a separate instrument, such as a thermometer.