(d) $i = \frac{e}{t} = \frac{e}{2\pi r/v} = \frac{ev}{2\pi r}$
Here, $v = \frac{e^{2}}{h}and\ r = h^{2}/me^{2}$
$\therefore i = \frac{e(\frac{e^{2}}{h})}{2\pi(h^{2}/me^{2})} = \frac{e^{3} \times me^{2}}{2\pi h^{3}} = \frac{me^{5}}{2\pi h^{3}}$
$i = \frac{4\pi^{2}me^{5}}{h^{3}}$