Correct answer is (d) S = 8d^2
Explanation: For BCC unit cell the relation between radius of a particle ‘r’ and edge length of unit cell, a, is r = \(\frac{\sqrt{3}}{4}\)a.
We know that diameter, d = 2r = \(\frac{\sqrt{3}}{2}\)a
Implying d^2 = \(\frac{3}{4}\)a^2
Therefore, 4d^2/3=a^2
Multiplying by 6 on both sides gives S = 6a^2 = 8d^2, where S is the surface area of the cube = 6a^2.