a
For maximum current,
$\mathrm{mR}=\mathrm{nr}$ ..........$(i)$
Given that $\mathrm{mn}=45$ ..........$(ii)$
Hence, $\quad \mathrm{m} \times 2.5=\mathrm{n} \times 0.5$
or $\mathrm{n}=5 \mathrm{\,m}$ .........$(iii)$
From equation $(ii)$ and $(iii),$ $5 \mathrm{m}^{2}=45$ or $\mathrm{m}=3$
$\Rightarrow \quad n=15$