1. What is aqua-regia?

Aqua-regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio: $3:1 \; (v/v)$ So, Option A is correct.

2. Why is aqua-regia yellow?

When conc. $HCl$ and conc. $HNO_3$ are mixed, they react to produce species such as nitrosyl chloride and chlorine: $HNO_3 + 3HCl \rightarrow NOCl + Cl_2 + 2H_2O$ Both $NOCl$ and $Cl_2$ contribute to the characteristic yellow/orange appearance of aqua-regia. Hence, Option B is correct.

3. Reaction of gold with aqua-regia

Gold dissolves in aqua-regia because:

• $HNO_3$ oxidizes $Au$ to $Au^{3+}$
• $Cl^-$ from $HCl$ stabilizes it as a chloro-complex

The overall reaction is commonly written as: $Au + HNO_3 + 4HCl \rightarrow HAuCl_4 + NO + 2H_2O$ The anion formed is: $[AuCl_4]^-$ Now, for oxidation state of Au in $[AuCl_4]^-$: Let oxidation state of Au be $x$. $x + 4(-1) = -1$ $x - 4 = -1$ $x = +3$ So, Option C is correct.

4. Does gold with aqua-regia produce $NO_2$ in absence of air?

From the standard reaction of gold with aqua-regia: $Au + HNO_3 + 4HCl \rightarrow HAuCl_4 + NO + 2H_2O$ The nitrogen oxide initially formed is $NO$, not $NO_2$. $NO$ may further oxidize in air to $NO_2$: $2NO + O_2 \rightarrow 2NO_2$ Therefore, in the absence of air, $NO_2$ is not produced. So, Option D is incorrect.

5. Final selection

Correct options are: $\boxed{A,\;B,\;C}$

6. Comparison with stored correct answer

Stored correct answer: $A, B, C$

My derived answer matches the stored answer.