Right answer is (b) Molality
To elaborate: The base of calculating molality of a solution is finding out the number of moles of solute per kg of solvent. Molarity is the number of moles o solute present per unit volume of solution, in liters. Molar mass refers to the mass of substance constituting one mole. Milli-molarity refers to number of milli-moles of solute present per unit volume of solution, in liters.