The most commonly used bleaching agent is

Bleaching agents are substances used to whiten or remove color.
Option A: Alcohol - Alcohol is a good solvent and disinfectant, but it doesn't effectively remove color.
Option B: Carbon Dioxide - Carbon dioxide is a gas we breathe out and is used in fire extinguishers and to make fizzy drinks. It doesn't have bleaching properties.
Option C: Chlorine - Chlorine is widely used as a bleaching agent because it's very effective at breaking down the chemical compounds that cause color. Think of cleaning products and swimming pools – chlorine is often used in those.
Option D: Sodium Chloride - Sodium chloride is actually common table salt (NaCl). It's important for our bodies but not a bleaching agent.
Therefore, chlorine is the most common and effective bleaching agent among the options provided.