Brushes of D.C. machines are made of

The correct answer is: A. Carbon

Brushes of D.C. machines are made of carbon because it is a good conductor of electricity and has a low resistance. It is also a good wear material and can withstand the high temperatures generated in a D.C. machine.

Soft copper is not a good choice for brushes because it is a poor conductor of electricity and has a high resistance. Hard copper is also not a good choice because it is a brittle material and can easily break.

Here is a brief explanation of each option:

• Carbon: Carbon is a good conductor of electricity and has a low resistance. It is also a good wear material and can withstand the high temperatures generated in a D.C. machine.
• Soft copper: Soft copper is a poor conductor of electricity and has a high resistance.
• Hard copper: Hard copper is a brittle material and can easily break.