Explanation

(d)
$a = \left( \sqrt[2]{2r} \right)\$Packing fraction
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{2\ \times \ \pi r^{2}\ }{\left( \sqrt[2]{2r}\ \right)^{2}} = \frac{2\pi r^{2}}{8r^{2}}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{\pi}{4} = \frac{3.14}{4} = 0.78\ 54$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 78.54\%$