Final answer:
A motor is indeed a mechanical device that converts electrical or other energy forms into mechanical energy, typically in the form of rotational motion. So, the correct option is A.
Step-by-step explanation:
The question concerns the functionality of a motor, which is a device that converts energy from one form to another. A motor is a mechanical driver with rotational output. It converts electrical energy into mechanical energy. When a current is passed through loops of wire in a magnetic field, the magnetic field exerts torque on the loops, causing them to rotate a shaft and convert electrical energy into mechanical work.
When discussing a motor, specifically an electric motor, it is accurate to state that it transforms electrical energy into mechanical energy. Additionally, regarding the efficiency of a simple machine, it is true that the efficiency is always less than 100 percent because some energy is inevitably converted to heat energy due to friction. Lastly, an external force is indeed defined as one that is generated outside of the system and then acts on an object within that system, making that statement true as well.