b
The balancing condition for meter bridge is,
$\frac{X}{R}=\frac{l_{1}}{100-l_{1}}$
$X=R\left(\frac{l_{1}}{100-l_{1}}\right)$
$12=18\left(\frac{l_{1}}{100-l_{1}}\right)$
$1200=30 l_{1}$
$l_{1}=40 \mathrm{cm}$
When $R=8$ ohm,
$12=8\left(\frac{l_{2}}{100-l_{2}}\right)$
$1200=20 l_{2}$
$l_{2}=60 \mathrm{cm}$
Hence, distance through $J$ is shifted is
$l=l_{2}-l_{1}=20 \mathrm{cm}$