To halve the molarity of a solution the following should be adopted

We know that Molarity$=\frac{\mathrm{No}. \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{solute} }{\mathrm{Volume} \mathrm{of} \mathrm{solution}(\mathrm{in} \mathrm{litres})}$

As we can see Molarity is inversely proportional to the volume of the solution, so to halve the molarity of a solution volume of the solution should be doubled.

Hence, option(d) is the answer.