In Van-de-Graff generator potential on the outer field increases at least until
A) The breakdown field of air is reached.
B) The threshold voltage of air is reached.
C) Both A and B.
D) None of these.

Hint: Van de Graff generator is an electrostatic generator that accumulates charge on the outer sphere through the moving belt. This generates high potential on the outer sphere. The Van de Graff generator is used in particle accelerators and it is also very much useful in explaining electrostatics. It produces thousands of volts.

Complete step by step answer:
(i) The Van de Graff generator is an electrostatic generator. It accumulates more charges on the outer hollow sphere in the generator. This creates a high potential for the sphere. This high potential causes corona discharge.
(ii) The air is usually an insulator. But when the high voltage or threshold voltage of the air is reached, the electric field produced by the threshold voltage causes the air to conduct. The electric field that causes the air to conduct is called the breakdown field. Hence the air conduct and this is called corona discharge.

Hence the correct option is C.

Additional information:

The given image is the Van-de-Graff generator. The Van-de-Graff generator consists of a hollow metallic sphere, upper electrode, lower electrode that is ground, a side belt with positive charges, a spherical device with a negative charge, a lower roller with a negative charge, and an electric comb.

Note:
The generator develops a high voltage direct current at low current levels. This is by the rotating belt which accumulates more charges on the outer hollow sphere. This high voltage causes the air to attain its threshold voltage level. And this voltage develops the breakdown field. This breakdown field causes the air to conduct the current partially. The Van de Graff generator produces thousands of volts.