Assertion :- The observed bond angle in `NH_3` molecule is less than `109^@ 28`'.
Reason :-In ammonia molecule, the bond pair-bond pair repulsion is more than the lone pair-bond pair repulsion

Options

A. If both Assertion and Reason are CORRECT and Reason is the CORRECT explanation of the Assertion
B. If both Assertion and Reason are CORRECT but Reason is not the CORRECT explanation of the Assertion.
C. If Assertion is CORRECT but Reason is INCORRECT.
D. If Assertion is INCORRECT but Reason is CORRECT.

Answer

To solve the question, we need to analyze the assertion and the reason provided:

### Step 1: Analyze the Assertion
The assertion states that the observed bond angle in the NH₃ (ammonia) molecule is less than 109° 28'.

- **Hybridization of NH₃**: Ammonia has an sp³ hybridization, which typically has a bond angle of 109° 28' in a perfect tetrahedral geometry.
- **Actual Geometry**: However, due to the presence of one lone pair of electrons on the nitrogen atom, the geometry is not perfectly tetrahedral. The lone pair occupies more space and pushes the bond pairs closer together, resulting in a bond angle that is less than 109° 28'.

**Conclusion for Step 1**: The assertion is correct; the bond angle in NH₃ is indeed less than 109° 28'.

### Step 2: Analyze the Reason
The reason states that in the ammonia molecule, the bond pair-bond pair repulsion is more than the lone pair-bond pair repulsion.

- **Repulsion Types**: In molecular geometry, lone pairs exert greater repulsion on bond pairs than bond pairs do on each other. This is because lone pairs are localized closer to the nucleus and occupy more space than bonding pairs.
- **Correct Understanding**: In NH₃, the lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion, which leads to a decrease in the bond angle.

**Conclusion for Step 2**: The reason provided is incorrect; the bond pair-bond pair repulsion is not greater than lone pair-bond pair repulsion.

### Final Conclusion
- The assertion is correct, but the reason is incorrect. Therefore, the correct option is: **Assertion is correct but reason is incorrect.**