Aqua regia is used for dissolving noble metals (Au, Pt, etc.). The gas evolved in this process is

First, recall that aqua regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the volume ratio $$3:1$$. Because both acids are present together, they react among themselves even before any metal is added.

We write the acid-acid reaction first. One molecule of $$\mathrm{HNO_3}$$ oxidises three molecules of $$\mathrm{HCl}$$:

$$$\mathrm{HNO_3 + 3\,HCl \;\longrightarrow\; NOCl + Cl_2 + 2\,H_2O}$$$

In this equation $$\mathrm{NOCl}$$ (nitrosyl chloride) and $$\mathrm{Cl_2}$$ are produced. The chlorine is in the nascent (freshly liberated) state and acts as a very strong oxidising as well as complex-forming species, which is why even noble metals get dissolved.

Now we bring a noble metal, say gold, into the mixture. Gold is oxidised by the nascent chlorine while chloride ions simultaneously form a complex with gold. The corresponding half-reactions are written below.

Oxidation (gold gives electrons):

$$\mathrm{Au \;\longrightarrow\; Au^{3+} + 3e^-}$$

Complex formation (gold(III) combines with chloride):

$$\mathrm{Au^{3+} + 4\,Cl^- \;\longrightarrow\; [AuCl_4]^-}$$

Nitrate from $$\mathrm{HNO_3}$$ is the species that accepts the electrons. We write its reduction half-reaction in acidic medium:

$$$\mathrm{NO_3^- + 4\,H^+ + 3e^- \;\longrightarrow\; NO + 2\,H_2O}$$$

Next, we add the two half-reactions, making sure the electrons cancel:

$$$\begin{aligned} \mathrm{Au} &\;\longrightarrow\; \mathrm{Au^{3+} + 3e^-} \\ \mathrm{NO_3^- + 4\,H^+ + 3e^-} &\;\longrightarrow\; \mathrm{NO + 2\,H_2O} \\ \hline \mathrm{Au + NO_3^- + 4\,H^+} &\;\longrightarrow\; \mathrm{Au^{3+} + NO + 2\,H_2O} \end{aligned}$$$

Now we incorporate the chloride ions that were already present in large excess from the $$3$$ parts of $$\mathrm{HCl}$$:

$$\mathrm{Au^{3+} + 4\,Cl^- \;\longrightarrow\; [AuCl_4]^-}$$

Adding this step gives the overall equation that chemists usually quote for the dissolution of gold in aqua regia:

$$$\mathrm{Au + NO_3^- + 4\,H^+ + 4\,Cl^- \;\longrightarrow\; [AuCl_4]^- + NO + 2\,H_2O}$$$

The single gaseous product appearing on the right-hand side of the balanced equation is $$\mathrm{NO}$$, nitric oxide. Although this $$\mathrm{NO}$$ quickly reacts with atmospheric oxygen to give brown fumes of $$\mathrm{NO_2}$$, the gas actually evolved at the instant of reaction from the solution is still $$\mathrm{NO}$$.

Therefore, among the four options given, the gas evolved when aqua regia dissolves noble metals is nitric oxide, symbolised as $$\mathrm{NO}$$.

Hence, the correct answer is Option A.