A solution of a substance X is used for white washing. What is X out of the following compounds?

CaO

The correct answer is option 1. \(CaO\).

The correct substance used for whitewashing is calcium oxide (\(CaO\)), also known as quicklime.

When calcium oxide is mixed with water, it undergoes a vigorous exothermic reaction to form calcium hydroxide (\(Ca(OH)_2\)). This process is known as slaking. The resulting calcium hydroxide is then applied to the surface to be whitewashed.

Over time, the calcium hydroxide absorbs carbon dioxide from the air and converts back into calcium carbonate (\(CaCO_3\)). This calcium carbonate forms a thin, white layer on the surface, resulting in a whitewashed appearance.

So, the correct answer is indeed: \(CaO\).