⇒Solid State

Correct Option is (D)

Number of mass $=\frac{\mathrm{Mass}}{\mathrm{Atomic}\mathrm{mass}}\times {\mathrm{N}}_{\mathrm{A}}=\frac{3.9}{39}\times {\mathrm{N}}_{\mathrm{A}}=0.1{\mathrm{N}}_{\mathrm{A}}$

In BCC unit cell, n = 2

$\therefore$ Number of unit cells $=\frac{0.1{\mathrm{N}}_{\mathrm{A}}}{2}=\frac{{\mathrm{N}}_{\mathrm{A}}}{20}$

Correct Option is (D)

The packing efficiency of a crystal structure refers to the fraction of the volume of a unit cell that is actually occupied by atoms. Let's look at the packing efficiencies of the given crystal structures :

Option A : FCC (Face-Centered Cubic)

Packing efficiency = 74%

Option B : BCC (Body-Centered Cubic)

Packing efficiency = 68%

Option C : Hexagonal cubic There's some confusion with this terminology. Typically, we have "Hexagonal Close-Packed (HCP)" rather than "Hexagonal Cubic." The packing efficiency for HCP is also about 74%, similar to FCC.

Option D : Simple cubic Packing efficiency = 52.4%

From the given options, the closest to 52.36% packing efficiency is :

Option D : Simple cubic.