$\mathrm{N}\mathrm{a}\mathrm{N}{\mathrm{O}}_3\stackrel{\hspace{1em}\mathrm{\Delta }\hspace{1em}}{\to }\underset{\mathrm{g}\mathrm{a}\mathrm{s}}{\mathrm{A}}+\mathrm{B}$

$\mathrm{A}+\mathrm{H}\mathrm{C}\mathrm{l}\stackrel{\hspace{1em}\mathrm{C}\mathrm{u}\mathrm{C}{\mathrm{l}}_2\hspace{1em}}{\to }\underset{\mathrm{g}\mathrm{a}\mathrm{s}}{\mathrm{C}}+{\mathrm{H}}_2\mathrm{O}$

Colour of $\text{C}$ and $\text{A}$ are respectively.

• A

  Colourless, reddish brown
• Correct answer
  B

  Greenish yellow, Colourless
• C

  Yellow, reddish brown
• D

  Colourless, Greenish yellow

Hence, the correct answer is option: 'B'.