Hint: Cottrel smoke precipitator is also known as Electrostatic precipitator or ESP and it is a type of Filtration device which is used in removing fine particles from a gas that is flowing in the device, and the fine particles here refers to the dust and smoke present in the gas.
As we know a flowing gas is imparted in the device, the principle of this device is it removes the fine particles using the force which is induced from electrostatic charges and this principle is the neutralization of charges on the colloidal particles. The smoke is a colloidal solution of solid particles such as carbon, arsenic compounds, dust, etc present in the air and the smoke before it comes out from the chimney, is passed through a chamber consisting of thin wires and plates which are placed vertically that varies from one centimeter to eighteen centimeters apart from each other and holds a charge that is opposite to the charge carried by the smoke or dust particles. The smoke or dust particles when comes in contact with the plates have a tendency to lose their charges and settle down in the form of a precipitate, and these particles settle down on the floor of the chamber.
So, the correct answer is Option A that is neutralization.
Note: Cottrel smoke Precipitator is best suited for the particles which have normal resistivity towards neutralization as particles with many high resistivities have some adverse effects, and the particles which have low resistance are very difficult to collect as they gradually gain or lose their charges.