In face centred cubic close packed structure octahedral sites are present at

Options

A. edge centres
B. face centres
C. body centre
D. A & C Both

Explanation

To determine where the octahedral sites are present in a face-centered cubic (FCC) close-packed structure, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the FCC Structure**:
- A face-centered cubic structure has atoms located at each of the corners of the cube and at the centers of each of the cube's faces.

2. **Identify the Types of Interstitial Sites**:
- In a cubic structure, there are two main types of interstitial sites: octahedral and tetrahedral. We are specifically interested in the octahedral sites for this question.

3. **Locate the Octahedral Sites**:
- **Edge Centers**: In an FCC structure, there is an octahedral site located at the center of each edge of the cube. Since there are 12 edges in a cube, there will be octahedral sites at each of these edges.
- **Body Center**: There is also one octahedral site located at the body center of the cube.

4. **Calculate the Contribution of Octahedral Sites**:
- Each edge contributes 1/4 of an octahedral site because each edge is shared by four adjacent cubes.
- Therefore, for the 12 edges:
\[ \text{Contribution from edges} = 12 \times \frac{1}{4} = 3 \]
- The body center contributes 1 full octahedral site.

5. **Total Number of Octahedral Sites**:
- Adding the contributions from the edges and the body center gives:
\[ \text{Total octahedral sites} = 3 (\text{from edges}) + 1 (\text{from body center}) = 4 \]

6. **Final Answer**:
- The octahedral sites in a face-centered cubic close-packed structure are present at the edge centers and the body center.

### Conclusion:
The correct answer is that octahedral sites are present at the edge centers and the body center.