Lightning is a process of:

While lightning happens, the electric discharge occurs between clouds and the earth. Electric discharge happens when the negative charges accumulate on the lower parts of the cloud, and a positive charge is induced on the ground. These negative and positive charges attract each other, which results in a sudden electric discharge. Due to the tremendous heat produced in the process of the discharge, the air molecules start glowing, which we called as lightning.

The correct answer is electric discharge.