c
Current in the circuit, \(I=\frac{\varepsilon}{R+r}\)

Potential difference across \(R\)

\({V=I R=\left(\frac{\varepsilon}{R+r}\right) R} \)

\({V=\frac{\varepsilon}{1+\frac{r}{R}}}\)

When \(R=0, V=0\)

\(R=\infty, V=\varepsilon\)