Which one of the following is a reduction reaction?

• A

  $2 \mathrm{Mg}(\mathrm{s})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow 2 \mathrm{MgO}(\mathrm{s})$
• B

  $\mathrm{S}(\mathrm{s})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{SO}_2(\mathrm{~g})$
• Correct answer
  C

  $2 \mathrm{HgO}(\mathrm{s}) \xrightarrow{\text { heat }} 2 \mathrm{Hg}(l)+\mathrm{O}_2(\mathrm{~g})$
• D

  $\mathrm{Mg}(\mathrm{s})+\mathrm{S}(\mathrm{s}) \longrightarrow \mathrm{MgS}(\mathrm{s})$

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