Assertion : Nitrogen in inactive at room temperature but becomes reactive at elevated temperature (on heating or in the presence of catalyst).
Reason : In nitrogen molecule, there is delocalization electrons.

Options

A. If assertion is true but the reason is false
B. If assertion is false but reason is true
C. If both assertion and reason are true and the reason is the correct explanation of assertion
D. If both assertion and reason are true and the reason is not the correct explanation of assertion

Answer

### Step-by-Step Solution

1. **Understanding the Assertion**:
- The assertion states that nitrogen is inactive at room temperature but becomes reactive at elevated temperatures or in the presence of a catalyst. This is true because nitrogen (N₂) has a very strong triple bond between the two nitrogen atoms, making it relatively inert under normal conditions.

**Hint**: Consider the strength of the bonds in nitrogen and how temperature affects molecular activity.

2. **Analyzing the Structure of Nitrogen**:
- Nitrogen has five valence electrons. To achieve a stable octet, two nitrogen atoms share three pairs of electrons, forming a strong triple bond (N≡N). This bond is very stable and requires significant energy to break.

**Hint**: Recall the concept of covalent bonding and how atoms achieve stability through electron sharing.

3. **Effect of Temperature and Catalysts**:
- At elevated temperatures or in the presence of a catalyst, the energy provided can weaken the triple bond. This allows nitrogen to react with other substances, such as oxygen, to form compounds like nitrogen oxides.

**Hint**: Think about how increasing temperature can provide energy to overcome activation barriers in chemical reactions.

4. **Understanding the Reason**:
- The reason given states that there is delocalization of electrons in the nitrogen molecule. This is incorrect. In the nitrogen molecule (N₂), the electrons are localized in the triple bond and are not delocalized. Delocalization typically occurs in molecules with resonance structures or in systems with conjugated double bonds.

**Hint**: Differentiate between localized and delocalized electrons and consider the structure of nitrogen.

5. **Conclusion**:
- The assertion is true: nitrogen is inactive at room temperature but becomes reactive at elevated temperatures or in the presence of a catalyst. The reason is false because there is no delocalization of electrons in the nitrogen molecule.

**Final Answer**: The assertion is true, but the reason is false. Therefore, the correct option is option number one.