Which of the following is more reliable?

The correct answer is Option (3) → Molality (m)

Molality is moles of solute per kilogram of solvent. Both quantities are masses (moles come from mass), and mass does not change with temperature, so molality has the same value at every temperature.

Units that use the volume of the solution change with temperature, because a liquid expands on heating and contracts on cooling. That makes molality the more reliable unit.

NCERT (2026-27 edition):
Expressing Concentration of Solutions – “… Each method of expressing concentration of the solutions has its own merits and demerits. Mass %, ppm, mole fraction and molality are independent of temperature, whereas molarity is a function of temperature. This is because volume depends on temperature and the mass does not.”