The correct answer is D. chlorine.
When water is mixed with bleaching powder, a chemical reaction occurs that produces chlorine gas. This gas is a greenish-yellow color and has a strong, irritating odor. It is also highly toxic and can be fatal if inhaled in large quantities.
The reaction between water and bleaching powder is as follows:
$CaOCl_2 + H_2O \to CaCl_2 + H_2O + Cl_2$
In this reaction, calcium hypochlorite (bleaching powder) reacts with water to form calcium chloride, water, and chlorine gas. The chlorine gas is produced as a result of the decomposition of calcium hypochlorite.
Chlorine gas is a very reactive gas and can react with a variety of substances. It can also be used to disinfect water and to bleach paper. However, it is important to use chlorine gas with caution, as it can be dangerous if inhaled in large quantities.
The other options are incorrect because they are not gases that are produced when water is mixed with bleaching powder. Carbon dioxide is a gas that is produced when carbon-containing compounds are burned. Hydrogen is a gas that is produced when water is electrolyzed. Oxygen is a gas that is present in the air.