Which one of the following is a reduction reaction

Options

A. `2Mg(s)+O_(2)rarr2Mg(s)`
B. `S(s)+O_(2)(g)rarrSO_(2)(g)`
C. `2HgO(s) rarr 2Hg(l)+O_(2)(g)`
D. `Mg(s)+S(s)rarrMgS(s)`

Explanation

To determine which of the given reactions is a reduction reaction, we need to analyze the changes in oxidation states of the elements involved in each reaction. A reduction reaction is characterized by a decrease in oxidation state, indicating that an element gains electrons.

### Step-by-Step Solution:

1. **Identify the Reactions**:
We are given several reactions. Let's analyze each one to determine the changes in oxidation states.

2. **Reaction 1: Mg + O2 → MgO**
- Magnesium (Mg) starts with an oxidation state of 0 and ends with +2 in MgO.
- Oxygen (O2) starts with an oxidation state of 0 and ends with -2 in MgO.
- **Change**: Mg goes from 0 to +2 (oxidation), O goes from 0 to -2 (reduction). However, since Mg is oxidized, this is not a reduction reaction.

3. **Reaction 2: S + O2 → SO2**
- Sulfur (S) starts with an oxidation state of 0 and ends with +4 in SO2.
- Oxygen (O2) starts with an oxidation state of 0 and ends with -2 in SO2.
- **Change**: S goes from 0 to +4 (oxidation), O goes from 0 to -2 (reduction). Again, this is not a reduction reaction for sulfur.

4. **Reaction 3: HgO → Hg + O2**
- Mercury (Hg) in HgO has an oxidation state of +2 and ends with 0 in elemental Hg.
- Oxygen (O) in HgO has an oxidation state of -2 and ends with 0 in O2.
- **Change**: Hg goes from +2 to 0 (reduction), O goes from -2 to 0 (oxidation). This indicates that Hg is being reduced.

5. **Reaction 4: Mg + S → MgS**
- Magnesium (Mg) starts with an oxidation state of 0 and ends with +2 in MgS.
- Sulfur (S) starts with an oxidation state of 0 and ends with -2 in MgS.
- **Change**: Mg goes from 0 to +2 (oxidation), S goes from 0 to -2 (reduction). However, since Mg is oxidized, this is not a reduction reaction.

6. **Conclusion**:
The only reaction where the oxidation state decreases (indicating a reduction) is **HgO → Hg + O2**. Therefore, this is the reduction reaction.

### Final Answer:
The reduction reaction is **HgO → Hg + O2**.