Animated Solution for Chemistry - s and p-Block Elements: Aqua-regia is used for dissolving noble metals (Au, Pt, etc.). The gas evolved in this process is

Aqua Regia

Imagine a liquid so powerful that it can dissolve the king of metals—gold. This legendary mixture is known as Aqua Regia, or "royal water." It is a highly corrosive, fuming yellow-orange liquid created by mixing two potent acids: concentrated hydrochloric acid ($\text{HCl}$) and concentrated nitric acid (${\text{HNO}}_3$) in a strict $3:1$ volume ratio.

Individually, neither of these acids can dissolve noble metals like gold ($\text{Au}$) or platinum ($\text{Pt}$). Nitric acid is a strong oxidizer, but it cannot form stable complexes with gold. Hydrochloric acid provides the necessary chloride ions to form stable complexes, but it lacks the oxidizing power. However, when combined, they create a chemical synergy that is truly remarkable.

When concentrated $\text{HCl}$ and ${\text{HNO}}_3$ are mixed, a vigorous reaction takes place. The nitric acid oxidizes the hydrochloric acid, producing a volatile and highly reactive mixture of products:

The key players generated here are nitrosyl chloride ($\text{NOCl}$) and chlorine gas (${\text{Cl}}_2$). But the reaction doesn't stop there. The nitrosyl chloride is unstable and further decomposes to yield nitric oxide gas and highly reactive nascent chlorine ($[\text{Cl}]$):

It is this nitric oxide ($\text{NO}$) gas that bubbles out of the solution during the process.

The nascent chlorine ($[\text{Cl}]$) produced in the mixture is incredibly aggressive. It attacks the noble metals, oxidizing them and allowing the chloride ions to form stable, soluble coordination complexes.

For gold, the reaction forms soluble chloroauric acid:

For platinum, the reaction forms soluble chloroplatinic acid:

Through this beautiful interplay of oxidation by nitric acid and complexation by hydrochloric acid, the noble metals are brought into solution. The primary gas that evolves and escapes the beaker during this fascinating dissolution process is nitric oxide ($\text{NO}$). Therefore, the correct answer to our question is NO.