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 \((ii)\)

\(r=1 \,\Omega, E=\frac{3}{2} \,V=\frac{15}{10}\, \text { volt }\)

\(\Rightarrow {x}=15\)