The correct option is C Charges flow easily between the clouds since air is a good conductor of electricity.
Clouds get charged because of the friction with the air molecules. When such a charged cloud approaches another cloud, it induces opposite charge in the second cloud. Since air is a bad conductor of electricity, it resists the flow of charge between the clouds. But when the magnitude of accummulated charge becomes very large, electric discharge takes place between clouds by breaking the resistive barrier of the air in between them. This discharge is seen as bright streaks in the form of lightning.