In the primitive 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 primitive 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 $r=a/2$.

Atoms in a primitive cubic unit cell touch all of their neighboring atoms, and the radius of an atom is equal to r.

$2\mathrm{r}=a$

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

Hence the option A is correct.