(b) cable of a microwave oven has three wires inside it which have insulation of different colours black, green and red. mention the significance of the three colours and potential difference between red and black one.

The green is ground.  Ideally, no current travels in this one.  The red and black are the power and neutral wires but which colors they are depends on a convention.  In the US, you will actually have a black (power) and a white (neutral)  Here it's red and black and usually in a red/black system the red is the power.  Either way there is a potential of 120V rms between them.

Ithink the answer would be a