c
Given, $emf$ of cell $\mathrm{E}=200 \,\mathrm{V}$
Internal resistance of cells $=1 \,\Omega$
D. C main supply voltage $V=220\, \mathrm{V}$
External resistance $\mathrm{R}=?$
$r=\left(\frac{E-V}{V}\right) R$
$\mathrm{1}=\left(\frac{20}{220}\right) \times \mathrm{R}$
$\mathrm{R}=11 \,\Omega$