A reduction reaction is a chemical reaction in which a substance gains electrons or reduces its oxidation state. The correct option, option 3, shows a reduction reaction. In this reaction, 2 HgO (solid) is being reduced to 2 Hg (liquid) and 02 (gas).

Option 1, on the other hand, does not involve any reduction reaction. It shows a combination reaction where 2 Mg (solid) and 02 (gas) combine to form 2 MgO (solid).

Option 2 is not a reduction reaction either. It shows a combustion reaction where S (solid) and 02 (gas) combine to form S02 (gas). The heat mentioned indicates the reaction is exothermic.

Option 4 also does not involve reduction. It shows a synthesis reaction where Mg (solid) and S (solid) combine to form MgS (solid).

Therefore, among the given options, only option 3 (2 HgO (solid) -> 2 Hg (liquid) + 02 (gas)) is a reduction reaction because mercury oxide (HgO) is being reduced to elemental mercury (Hg).