To understand why both inlet and exhaust valves remain closed during the power stroke of a diesel engine, we need to analyze the engine's operation process, specifically during the power stroke.

1. Understanding the Power Stroke: In a diesel engine, the power stroke occurs after the air-fuel mixture inside the cylinder has been ignited. The combustion of the fuel results in high-pressure gases being produced.
2. Role of Valves: The inlet and exhaust valves are responsible for controlling the entry and exit of gases in the cylinder. During the power stroke, both of these valves remain closed.
3. Reason for Keeping Valves Closed: The main purpose of keeping both valves closed is to ensure that the high-pressure combustion gases are confined within the cylinder. This confinement allows the energy from these gases to exert maximum force on the piston, pushing it down the cylinder, which converts the energy into mechanical work.
4. Option Analysis:
   • Option a: To confine high-pressure combustion gases and convert their energy into useful work - This option correctly explains the reason, as confining the gases allows their energy to be utilized effectively to drive the piston.
   • Option b: To increase the duration of fuel injection before the piston reaches Bottom Dead Centre - This is incorrect because the timing of fuel injection is typically controlled separately and isn't directly related to valve positions during the power stroke.
   • Option c: To improve lubrication of the valve guides by reducing exhaust gas temperature - Incorrect, as valve lubrication is managed through oil flow and is not the function of keeping valves closed during the power stroke.
   • Option d: To allow maximum heat transfer from the cylinder head to the piston crown - This is also incorrect since heat transfer specifics don't dictate the valve positions.
5. Conclusion: Hence, the correct explanation for keeping both valves closed during the power stroke is to confine high-pressure combustion gases and convert their energy into useful work. This is aligned with the primary operation of the power stroke in an engine and ensures maximum efficiency in energy conversion.