Introduction:
A closed loop system is a control system in which the output is measured and compared to the desired input in order to make necessary adjustments. It continuously monitors the output and makes corrections to maintain the desired output. In this context, we will discuss the options given and identify which one represents a closed loop system.

Options:
a) Electric switch: An electric switch is a simple device that opens or closes an electrical circuit. It does not have any feedback mechanism to monitor the output and make adjustments. Therefore, it is not a closed loop system.

b) Car starter: A car starter is responsible for initiating the combustion process in an internal combustion engine. It is an electromechanical device that operates based on user input. It does not have a feedback mechanism to monitor the output and make adjustments. Therefore, it is not a closed loop system.

c) DC generator: A DC generator converts mechanical energy into electrical energy. It does not have any feedback mechanism to monitor the output and make adjustments. Therefore, it is not a closed loop system.

d) Auto-pilot for an aircraft: An auto-pilot system in an aircraft is designed to automatically control the flight path and maintain stability during flight. It continuously monitors various parameters such as altitude, airspeed, and heading, and makes adjustments to the control surfaces to maintain the desired flight parameters. This feedback mechanism allows the system to continuously correct for any deviations from the desired flight path, making it a closed loop system.

Conclusion:
Based on the above analysis, the correct answer is option 'd' - Auto-pilot for an aircraft. It is the only option among the given choices that involves a closed loop system where the output is continuously monitored and adjusted to maintain the desired input.