a
The internal resistance of the cell is
$r=\left(\frac{l_{1}}{l_{2}}-1\right) R$
Here, $l_{1}=3\, \mathrm{m}, l_{2}=2.85\, \mathrm{m}, R=9.5\, \Omega$
$\therefore \quad r=\left(\frac{3}{2.85}-1\right)(9.5 \,\Omega)=\frac{0.15}{2.85} \times 9.5\, \Omega=0.5\, \Omega$