The graph showing the variation of the magnetic field strength B...

Magnetic field strength $B$ due to long straight current carrying conductor at a distance $r$ is given as $$\begin{aligned} B \text{ and } =\frac{\mu_{0}}{2 \pi} \cdot \frac{I}{r}\\ \Rightarrow \quad \text{ and } B \propto \frac{1}{r} \end{aligned}$$ Hence, correct variation of $B$ with $r$ is shown in the graph given in option (c).