Which of the following reactions is a neutralisation reaction?

Options

A. `4 Na + O_(2) to 2Na_(2)O`
B. `Fe + 2HCl to FeCl_(2) + H_(2)`
C. `MgO + H_(2)O toMg(OH)_(2)`
D. `HNO_(3) + NaOH to NaNO_(3) + H_(2)O`

Answer

To determine which of the given reactions is a neutralization reaction, we need to understand the definition of a neutralization reaction. A neutralization reaction occurs when an acid reacts with a base to produce a salt and water.

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to analyze each of the given reactions to see if they fit the definition of a neutralization reaction.

2. **Examine the First Reaction**:
- **Reaction**: Sodium (Na) + Oxygen (O2) → Na2O
- **Analysis**: This reaction involves the combination of sodium and oxygen to form sodium oxide. It does not involve an acid or a base, so it is not a neutralization reaction. This is a combination reaction.

**Hint**: Look for the presence of an acid and a base in the reaction.

3. **Examine the Second Reaction**:
- **Reaction**: Iron (Fe) + Hydrochloric Acid (HCl) → Iron(II) Chloride (FeCl2) + Hydrogen (H2)
- **Analysis**: In this reaction, iron displaces hydrogen from hydrochloric acid, forming iron(II) chloride and hydrogen gas. This is a displacement reaction, not a neutralization reaction, because it does not involve a base.

**Hint**: Check if the reaction produces water and a salt, which are characteristic of neutralization.

4. **Examine the Third Reaction**:
- **Reaction**: Magnesium Oxide (MgO) + Water (H2O) → Magnesium Hydroxide (Mg(OH)2)
- **Analysis**: This reaction shows magnesium oxide reacting with water to form magnesium hydroxide. This is also a combination reaction and does not involve an acid reacting with a base to produce salt and water.

**Hint**: Remember that neutralization reactions specifically involve an acid and a base.

5. **Examine the Fourth Reaction**:
- **Reaction**: Nitric Acid (HNO3) + Sodium Hydroxide (NaOH) → Sodium Nitrate (NaNO3) + Water (H2O)
- **Analysis**: Here, nitric acid (an acid) reacts with sodium hydroxide (a base) to produce sodium nitrate (a salt) and water. This fits the definition of a neutralization reaction.

**Hint**: Look for the formation of both a salt and water in the products.

### Conclusion:
The reaction that represents a neutralization reaction is:
**HNO3 + NaOH → NaNO3 + H2O**

Thus, the correct answer is **Option D**.