In the face-centred cubic unit cell of closest packed atoms, the radius of atom (r) is related to the edge length (a) of unit cell by the expression

In the face-centered cubic unit cell of closest packed atoms, the radius of atom (r) is related to the edge length (a) of unit cell by the expression is $r=a/2\sqrt{2}$.

 For the face-centered cubic unit cell the following ratio of the radius and edge length is obtained,

$\frac{r}{a}=2\sqrt{2}$

The radius of the atom is,

$r=a/2\sqrt{2}$

Hence the option C is correct.