Concentrated hydrochloric acid when kept in open air sometimes produces a cloud of white fumes.The correct reason for this is?
(A) Strong affinity of $HCl$ gas for moisture in air results in forming of droplets of liquid solution which appears like a cloudy smoke
(B) Due to strong affinity for water, concentrated $HCl$ pulls moisture from air towards itself. The moisture forms droplets of water and hence the cloud
(C) Concentrated $HCl$ emits strongly smelling $HCl$ gas all the time
(D) Oxygen in air reacts with emitted $HCl$ gas to form a cloud of $C{l_2}$ gas.

Hint:. Remember Concentrated hydrochloric acid when kept in open air sometimes produces a cloud of white fumes. Also Concentrated $HCl$ emits strongly smelling gas all the time. Lets now see the correct solution.

Complete answer: You know that Concentrated hydrochloric acid (36.5-38.0%) is a colorless to yellowish pungent liquid. Hydrochloric acid is historically called muriatic acid. Hydrochloric acid is commonly used to make aqua regia, typically 1 part Nitric acid to 3 parts Hydrochloric acid.Hydrochloric acid is frequently used in chemical analysis to prepare ("digest") samples for analysis. Concentrated hydrochloric acid dissolves many metals and forms oxidized metal chlorides and hydrogen gas.The $HCl$ react with the air that is oxygen undergoes the reaction:$4HCl + {O_2} \to 2C{l_2} + 2{H_2}O$
Where $2C{l_2} + 2{H_2}O$ is the cloud of white fumes.We can say that Concentrated hydrochloric acid when kept in open air sometimes produces a cloud of white fumes because Oxygen in air reacts with $HCl$ emitted gas to form a cloud of $C{l_2}$ gas.

Hence (D) is the correct option.

Note:Understand that Chronic or prolonged exposure to hydrogen chloride gas or to mist has been associated with changes in pulmonary function, chronic inflammation of the bronchi, nasal ulceration, and symptoms resembling acute viral infection of the upper respiratory tract as well as inflammation of the skin, discoloration and erosion of dental enamel, and inflammation of the eye membrane.