Hello,

In Face Centered Cubic (FCC) Lattice, the atoms are present at the corners, unit cell as well as on the face centers of the cubic. Also, the atoms on the face diagonal will be touching each other. The edge of the cube is ‘a’ (given)

Let the radius be ‘r’

Now, face diagonal of the cube = root(2)*a

Therefore, r + 2r + r = root(2)*a

Solving this equation, we get r = a/2*root(2)

Now, diameter is equal to twice of radius = 2 * r

Therefore, diameter of copper atom will be: 2*a/(2*root(2))

= a/(root(2))

Therefore, third option is correct.

Hope this helps!