Yellow Brown Coloration of Concentrated Nitric Acid on Standing

Pure concentrated nitric acid ($\text{HNO}_3$) is a colorless liquid. However, upon exposure to light or upon long standing, it undergoes slow photochemical and thermal decomposition according to the following reaction:

$4\text{HNO}_3 \xrightarrow{h\nu \text{ or } \Delta} 4\text{NO}_2 + 2\text{H}_2\text{O} + \text{O}_2$

The nitrogen dioxide ($\text{NO}_2$) gas formed is reddish-brown in color. As it dissolves in the remaining concentrated nitric acid, it imparts a characteristic yellow to yellow-brown coloration to the acid.

Therefore, the coloration is primarily due to the formation of $\text{NO}_2$.

Correct Option: (B)