A solid has a bcc structure. If the distance of closest approach between the two atoms is 1.73oA. The edge length of the cell is:
In bcc structure, the atoms are present at corners and at body center and the atoms along the body diagonal are touching each other.
So, if a is the edge length of the cube and r is the radius of the atom.
Now, distance of closest approach is reached when two atoms are touching each other i.e their centers are at 2r distance from each other.
So, √2a=1.73 ˚A
Option A is correct.