What is the unit cell generated in the hexagonal packing in two dimension?

A metal $\mathrm{M}$ forms hexagonal close-packed structure. The total number of voids in $0.02 \mathrm{mol}$ of it is _____ $\times {10}^{21}$ (Nearest integer)

(Given ${\mathrm{N}}_{\mathrm{A}}=6.02\times {10}^{23}$)

In a face centered cubic (fcc) lattice, the position which represents an octahedral hole is

A two dimensional solid is made by alternating circles with radius $\mathrm{a}$ and $\mathrm{b}$ such that the sides of the circles touch. The packing fraction is defined as the ratio of the area under the circles to the area under the rectangle with sides of length $\mathrm{x}$ and $\mathrm{y}.$

The ratio $\mathrm{r}=\mathrm{b}/\mathrm{a}$ for which the packing fraction is minimized is closest to: