In any graph of molar conductivity vs C, a weak electrolyte gives a curved decreasing graph strong electrolytes give straight-line decreasing graphs, in which the graph of stronger electrolyte will always be above the graph of the comparatively weaker electrolyte.
In option [B], Graph III is curved and corresponds to NH4OH which is a weak electrolyte. Graph II \& I respectively represent NaCl and HCl. (∵HCl is a stronger electrolyte among the two).