The correct answer is: C. 30 m/s
The peripheral speed of a D.C. machine is the speed at which the outer edge of the rotor rotates. It is calculated by multiplying the rotor’s diameter by its rotational speed. The peripheral speed should not, normally, exceed 30 m/s, as this could cause the rotor to overheat and damage the machine.
Option A: 10 m/s is too low a value for the peripheral speed of a D.C. machine. This would result in the machine not being able to produce enough power.
Option B: 20 m/s is a safe value for the peripheral speed of a D.C. machine. However, it is not the maximum value that can be used.
Option C: 30 m/s is the maximum value for the peripheral speed of a D.C. machine. If this value is exceeded, the rotor could overheat and damage the machine.
Option D: 40 m/s is too high a value for the peripheral speed of a D.C. machine. This would result in the rotor overheating and damaging the machine.