b
Terminal voltage $v=i R=\frac{E R}{R+r}$
$1^{s t} \rightarrow 1.25=\frac{E(5)}{5+r} . .(i)$
$2^{n d} \rightarrow 1=\frac{E(2)}{2+r}\, \cdots \text { (ii) }$
By $(i)$ and $(ii)$
$r=1 \,\Omega, E=\frac{3}{2} \,V=\frac{15}{10}\, \text { volt }$
$\Rightarrow {x}=15$
