Bleaching powder, also known as calcium hypochlorite, is a chemical compound that is widely used as a bleaching agent and disinfectant. It reacts with a few drops of dilute hydrochloric acid (HCl) to produce chlorine gas (Cl2). Let's understand the reaction in detail:

1. Chemical equation:
Ca(OCl)2 + 2HCl → CaCl2 + H2O + Cl2

2. Reactants:
- Bleaching powder (Ca(OCl)2): It contains calcium hypochlorite, which is the active ingredient. It is a yellowish-white powder that is highly soluble in water.
- Dilute hydrochloric acid (HCl): It is a strong acid commonly used in laboratories. It is a clear, colorless liquid with a pungent odor.

3. Reaction process:
- When bleaching powder is added to dilute hydrochloric acid, the following reaction takes place:
Ca(OCl)2 + 2HCl → CaCl2 + H2O + Cl2
- The calcium hypochlorite in bleaching powder reacts with hydrochloric acid to form calcium chloride (CaCl2), water (H2O), and chlorine gas (Cl2).
- This reaction is a redox reaction, where calcium hypochlorite acts as the reducing agent by losing electrons, and hydrochloric acid acts as the oxidizing agent by gaining electrons.

4. Chlorine gas production:
- The main product of the reaction is chlorine gas (Cl2).
- Chlorine gas is a greenish-yellow gas with a strong odor and is highly toxic.
- It is widely used in water treatment, disinfection, and as a bleaching agent for textiles, paper, and other materials.

Therefore, when bleaching powder reacts with a few drops of dilute hydrochloric acid, it produces chlorine gas as the main product. Chlorine (option A) is the correct answer.