The correct answer is A. gear trains.
Gear trains are a series of gears that are used to transmit power from one part of a machine to another. They can be used to increase or decrease the speed of a motor, as well as to change the direction of rotation.
V-belts are used to transmit power between two shafts that are not aligned. They are made of a flexible material, such as rubber, and have a V-shaped cross-section. The V-shape helps to grip the shafts and ensures that the belt stays in place.
Brakes are used to stop or slow down a machine. They can be applied to the motor itself, or to the wheels of the machine.
Chains are used to transmit power between two shafts that are not aligned. They are made of a series of links that are connected together. The links are usually made of metal, but can also be made of plastic or other materials.
In the case of a universal motor, the speed is generally reduced by using gear trains. This is because gear trains can provide a more precise reduction in speed than other methods, such as V-belts or chains.