The correct answer is: B. varying the resistance
A universal motor is a type of electric motor that can run on either AC or DC power. It is commonly used in sewing machines, vacuum cleaners, and power tools. The speed of a universal motor is controlled by varying the resistance in the circuit. This can be done by using a rheostat, a variable resistor.
Option A is incorrect because friction is not a reliable way to control the speed of a motor. Friction can vary depending on the surface of the materials in contact, and it can also be affected by wear and tear.
Option C is incorrect because tapping the field is not a common way to control the speed of a motor. Tapping the field is a technique that is used to increase the torque of a motor, but it does not have a significant effect on the speed.
Option D is incorrect because a centrifugal mechanism is not a common way to control the speed of a motor. A centrifugal mechanism is a device that uses centrifugal force to control the speed of a motor, but it is not typically used in sewing machines.
In conclusion, the speed of a universal motor is controlled by varying the resistance in the circuit. This can be done by using a rheostat, a variable resistor.