Clouds get electrically charged when:
A) The UV rays of the sun fall on them.
B) Air molecules rub against water molecules.
C) Cold currents of air rub against hot currents of air.
D) Both B and C.

Hint:The clouds form thunderstorms when two or more electrically charged clouds collide and get discharged. The static electrical charges are the electrical charges which do not move and they are formed by rubbing of the material.

Complete step by step solution:
It is asked in the problem of how the clouds get electrically charged.

The sun vaporize the water bodies and the components of the water now when this water goes up in the sky then it forms clouds and these clouds get rubbed by the air molecules in the sky and also by the water molecules present in the air and also by the water vapor due to which there is formation of static electrical charges due to the friction. So due to rubbing of hot air against the cold air and also by the rubbing of the air molecules with the water molecules the clouds get electrically charged.

The correct answer for this problem is option B and C so the correct option is option D.

Note:The clouds are the cluster formation due to the water vapors and as this cluster gets cool then the water molecules gets converted into the water molecules and gets back to the earth surface and at the same time the supercooled water becomes ice crystals and these ice crystals are electrically charged and when they collide then the thunderstorm happens. The cloud formation is the part of the water cycle which is one of the basic cycles of the earth and is very much required for life to sustain.