A : Number of rectangular plane in a cubic crystal is 3.
R : Rectangular planes passes through corner to corner of unit cell.

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 explanation of the assertion, then mark (2)
C. If Assertion is true statement 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 the statements A and R regarding the number of rectangular planes in a cubic crystal and their orientation.

### Step-by-Step Solution:

1. **Understanding the Statement A**:
- The statement A claims that the number of rectangular planes in a cubic crystal is 3.
- In a cubic crystal, we can visualize the unit cell as a cube. Each face of the cube can be considered a plane. However, when we talk about rectangular planes, we need to consider planes that are not just the faces but also those that can pass through the cube diagonally.

2. **Visualizing the Cubic Crystal**:
- Draw a cube to represent the unit cell of the cubic crystal.
- Identify the corners of the cube. The corners are the vertices where the edges of the cube meet.

3. **Identifying Rectangular Planes**:
- A rectangular plane can be defined as a plane that intersects the cube in such a way that it forms a rectangle.
- In a cubic structure, we can find three distinct rectangular planes that can be formed by connecting the midpoints of the edges of the cube.

4. **Confirming the Number of Rectangular Planes**:
- The three rectangular planes can be visualized as:
- One plane passing through the center of the cube and connecting the midpoints of opposite edges.
- The second plane can be oriented differently but still connecting midpoints of edges.
- The third plane will also connect different midpoints, ensuring that all three planes are distinct and rectangular.

5. **Understanding Statement R**:
- The statement R claims that rectangular planes pass through corner to corner of the unit cell.
- While the rectangular planes do connect various points within the cube, they do not necessarily pass from corner to corner. Instead, they pass through the midpoints of edges and faces.

6. **Evaluating the Truth of Statements**:
- Since statement A is true (there are indeed 3 rectangular planes in a cubic crystal), but statement R is false (the planes do not pass from corner to corner), we conclude:
- A is true.
- R is false.

7. **Final Conclusion**:
- The correct option based on the evaluation is that A is true, but R is false. Therefore, the answer is option C.