Which of the following ion is expected to have highest value of molar conductivity at infinite dilution in the aqueous solution?

At infinite dilution, ionic molar conductivity depends on ionic mobility in water. Among common cations, $\mathrm{H^+}$ has exceptionally high mobility because it moves through the hydrogen-bonded network of water by proton hopping. So the highest limiting molar conductivity is for $\mathrm{H^+}$. Final check against the options: $\mathrm{Na^+}$, $\mathrm{K^+}$, $\frac{1}{2}\mathrm{Ca^{2+}}$, and $\mathrm{H^+}$. The matching option is $\mathrm{H^+}$, which is option $\mathrm{D}$.