Which one of the following involves decrease in entropy?

• A

  $2 \mathrm{NaHCO}_{3(s)} \longrightarrow \mathrm{Na}_2 \mathrm{CO}_{3(s)}+\mathrm{CO}_{2(g)}+\mathrm{H}_2 \mathrm{O}_{(g)}$
• B

  $\mathrm{CCl}_{4(s)} \longrightarrow \mathrm{CCl}_{4(l)}$
• C

  $\mathrm{Zn}_{(s)}+2 \mathrm{HCl}_{(a q)} \longrightarrow \mathrm{ZnCl}_{2(a q)}+\mathrm{H}_{2(g)}$
• Correct answer
  D

  $\begin{aligned} & \mathrm{CaO}_{(s)}+\mathrm{CO}_{2(g)} \longrightarrow \mathrm{CaCO}_{3(s)} \end{aligned}$

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