The correct answer is: B. Electric discharge
Lightning is a sudden electrostatic discharge that occurs between a cloud and the ground or between two clouds. It is caused by the buildup of electrical charge in the atmosphere. When the charge becomes strong enough, it discharges in the form of a lightning bolt.
Electric power is the rate at which electrical energy is transferred by an electric circuit. It is measured in watts.
Electric leak is a flow of electric current through an unintended path. It can occur when there is a break in the insulation of an electrical wire or when there is a contact between two conductors of different potentials.
Electric pressure is the force per unit area exerted by an electric field. It is measured in volts per meter.
In conclusion, lightning is caused by an electric discharge, not by electric power, electric leak, or electric pressure.