The correct answer is C. Adsorption.
Adsorption is a surface phenomenon in which molecules or ions from a solution or gas phase adhere to a solid surface. The surface of the solid is called the adsorbent and the substance that is adsorbed is called the adsorbate.
Activated charcoal is a form of carbon that has been processed to increase its surface area. This makes it very effective at adsorbing a wide range of substances, including colouring matter. When activated charcoal is added to a solution containing colouring matter, the colouring matter is adsorbed onto the surface of the charcoal. This removes the colouring matter from the solution, making it clear.
Bleaching is a process that removes colour from a substance. This can be done by a variety of methods, including oxidation, reduction, and hydrolysis. Bleaching is often used to whiten fabrics and paper.
Oxidation is a chemical reaction in which an atom or molecule loses electrons. This can result in a change in the chemical properties of the substance. Oxidation is often used to clean and disinfect surfaces.
Reduction is a chemical reaction in which an atom or molecule gains electrons. This can result in a change in the chemical properties of the substance. Reduction is often used to produce metals from their ores.
D. Reduction is not the correct answer because activated charcoal does not reduce colouring matter.