A brown colored mixture of two gases is obtained by the reduction of 6N nitric acid with metallic copper. This mixture on cooling condense to a blue liquid which on freezing (30°) gives a blue solid. The correct choice for blue liquid or solid is:

Reduction of 6N HNO₃ with metallic copper is:

$2\mathrm{Cu}+6{\mathrm{HNO}}_3\rightleftharpoons 2\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2+\mathrm{NO}+{\mathrm{NO}}_2+3{\mathrm{H}}_2\mathrm{O}$

Two gaseous products formed from reduction of 6N HNO₃ is NO (colorless) and NO₂ (brown color). 

Formation of blue color solution at low temperature is:

$\mathrm{NO}+{\mathrm{NO}}_2\stackrel{-20°\mathrm{C}}{\rightleftharpoons }\underset{\mathrm{Blue}}{{\mathrm{N}}_2{\mathrm{O}}_3}$

It is anhydride of nitrous acid.

$2{\mathrm{HNO}}_2\to {\mathrm{N}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$ 

It dissolve alkali and produces nitrites.

${\mathrm{N}}_2{\mathrm{O}}_3+2\mathrm{KOH}\to 2{\mathrm{KNO}}_2+{\mathrm{H}}_2\mathrm{O}$

It is also prepared from arsenious oxide.

$2{\mathrm{NHO}}_3+{\mathrm{As}}_2{\mathrm{O}}_3+2{\mathrm{H}}_2\mathrm{O}\to \underset{\stackrel{ \downarrow 250 \mathrm{K}}{{\mathrm{N}}_2{\mathrm{O}}_3}}{\underbrace{\mathrm{NO}+{\mathrm{NO}}_2}}+2{\mathrm{H}}_3{\mathrm{AsO}}_4$

When the above formed mixture cools to 250 K, it forms N₂O₃.