A dynamo is a device that converts mechanical energy into electrical energy. Mechanical energy refers to the energy possessed by an object due to its motion or position, while electrical energy is the energy associated with the flow of electric charge. In a dynamo, a rotating coil of wire is placed in a magnetic field. As the coil rotates, it cuts through the magnetic field lines, which induces an electrical current in the wire. This current can be used to power electrical devices. Therefore, option A is the correct answer.