Step 1: Understanding the body centered cubic lattice.
In a body centered cubic (BCC) lattice, the relationship between the edge length \( a \) and the radius \( r \) of the atom is given by the formula:
\[
\sqrt{3}r = 4a
\]
This comes from the geometry of the lattice, where the diagonal of the cube is equal to \( 4r \), and the body diagonal is related to the edge length by \( \sqrt{3}a \).
Step 2: Solving for the radius.
Rearranging the formula:
\[
r = \frac{\sqrt{3}}{4} \times a
\]
Thus, the correct answer is (B).