The correct answer is: C. Variable speed operation.
DC motors are preferred over AC motors for variable speed operation because they are more efficient and have a wider speed range. DC motors can be controlled by adjusting the voltage or current, which allows for precise speed control. AC motors, on the other hand, are typically only able to be controlled by adjusting the frequency, which provides less precise speed control.
In addition, DC motors are typically smaller and lighter than AC motors, which makes them easier to install and maintain. They are also less expensive than AC motors.
However, AC motors have some advantages over DC motors. They are more rugged and can withstand higher temperatures. They are also easier to start and stop.
Overall, DC motors are the preferred choice for applications that require variable speed operation. They are more efficient, have a wider speed range, and are smaller and lighter than AC motors.