A : Dihedral angle of `H_2O_2` in gas phase is greater than in solid phase.
R : `H_2O_2` has planar structure.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanantion of the assertion, then mark (2)
C. If assertion is true statements but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) given in the statement about hydrogen peroxide (H₂O₂).

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the dihedral angle of H₂O₂ in the gas phase is greater than in the solid phase.
- Dihedral angle refers to the angle between two planes formed by four atoms. For H₂O₂, this involves the arrangement of the oxygen and hydrogen atoms.

2. **Analyzing the Dihedral Angles**:
- In the gas phase, the dihedral angle of H₂O₂ is reported to be approximately 111.5 degrees.
- In the solid phase, the dihedral angle is approximately 90.2 degrees.
- Since 111.5 degrees (gas phase) is greater than 90.2 degrees (solid phase), the assertion is **true**.

3. **Understanding the Reason (R)**:
- The reason states that H₂O₂ has a planar structure.
- To evaluate this, we need to consider the molecular geometry of H₂O₂. The structure of hydrogen peroxide is often described as an "open book" configuration, which indicates that it is not planar.

4. **Hybridization and Structure**:
- The oxygen atoms in H₂O₂ are sp³ hybridized due to the presence of two lone pairs on each oxygen atom. This hybridization leads to a tetrahedral arrangement around each oxygen, contributing to a non-planar structure.
- Therefore, the reason that H₂O₂ has a planar structure is **false**.

5. **Conclusion**:
- The assertion is true, and the reason is false. Thus, the correct option is that the assertion is true but the reason is false.

### Final Answer:
- The correct option is: **Assertion is true but reason is false.**