When an article is bleached by $S{O_2}$ it loses its color. The color can be restored by:
A. Exposure to air
B. Heating
C. Dilution
D. None of these

Hint: Bleaching is the process of removing color from a fiber or a fabric. It is done as a part of cleaning or removing stains in industrial and domestic uses. The agents that provide the bleaching effect are known as Bleaching agents

Complete step by step answer:
Bleaching agents primarily oxidizing agents, while only a handful of bleaching agents exist that are reducing agents. $S{O_2}$ is a bleaching compound that acts by chemical reduction. The bleaching action of $S{O_2}$ is temporary and can be reversed by exposing the article to air, here the color that got light due to bleaching is restored by the process of oxidation. The chemical reactions involved in the process are:
$S{O_2}$ exhibit reducing property under the presence of water, the reaction that follows is:

$S{O_2} + 2{H_2}O \to {H_2}S{O_4} + 2{H^ + }$,

The ${H^ + }$ ions obtained in the above process are responsible for bleaching action and when the article is exposed to the air these ${H^ + }$ oxidize to give water thus restoring the color.

So, the correct answer is Option A.

Note: The majority of Bleaching agents work as oxidation agents and the bleaching action by them is irreversible. Some of the examples of such bleaching agents are Chlorine, Hydrogen peroxide, ozone. Sodium Perborate is the most commonly used bleaching agent.

Bleaching agents tend to lighten the color by making the substrate more soluble and also It might alter its light-absorbing capability. Textile, pulp, and paper industry are major industries that utilize bleaching agents. Some of the examples of bleaching agents that act as reducing agents are Sulphur dioxide, Sodium tetrahydroborate, sodium hypochlorite.