Volume of the domain, V=a3
V=10−12cm=10−18m
Density, ρ=7.9g/cm3
Mass of domain, M=V×ρ
M=10−12×7.9
M=7.9×10−12g
The number of atom in domain,
n=MNAm
n=7.9×10−12×6.023×102455g
n=8.65×1010atom
The total dipole moment, P=np
P=8.65×1010×9.27×10−24
P=8×10−13A−m2
The magnetization of the domain, M=PV
M=8×10−1310−18
M=8×105A/m
The correct option is A.