Assertion:Packing efficiency of body centred cubic structure is 68%
Reason:68% is the maximum packing efficiency any crystal can have

Options

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

Explanation

To solve the question, we will analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the packing efficiency of a body-centered cubic (BCC) structure is 68%.
- Packing efficiency is defined as the fraction of volume in a crystal structure that is occupied by the constituent particles (atoms, ions, or molecules).

2. **Verifying the Assertion**:
- For a body-centered cubic structure, the packing efficiency can be calculated as follows:
- In a BCC unit cell, there are 2 atoms per unit cell (1 atom at the center and 1/8 of an atom at each of the 8 corners).
- The packing efficiency for BCC is indeed calculated to be approximately 68%.
- Therefore, the assertion is **correct**.

3. **Understanding the Reason**:
- The reason states that 68% is the maximum packing efficiency any crystal can have.
- This statement is incorrect because there are crystal structures with higher packing efficiencies.

4. **Verifying the Reason**:
- The face-centered cubic (FCC) structure has a packing efficiency of approximately 74%, which is higher than that of BCC.
- Therefore, the reason is **incorrect**.

5. **Conclusion**:
- The assertion is true, but the reason is false. Thus, the overall statement that both the assertion and reason are true is incorrect.

### Final Answer:
- The assertion is correct, but the reason is incorrect.