To change the molal conc. to one half, one of the following should be adopted

We know that molality$=\frac{Number of moles of solute}{Weight of solvent(in kg)}$

So, we can see molality is directly proportional to number of moles of solute and inversely proportional to weight of solvent.

So, to change the molal concentration to one half, weight of solvent should be doubled.

Hence, option(b) is correct.