Explanation:
The molecular weight of a substance refers to the mass of one molecule of that substance relative to the mass of one hydrogen atom. It is an important parameter in chemistry as it helps in determining the stoichiometry of chemical reactions and understanding the properties of substances.

Density in the liquid state:
The density of a substance in the liquid state can provide information about its mass per unit volume. However, it does not directly provide information about the molecular weight of the substance. The density can vary depending on the temperature and pressure conditions, and it is influenced by factors such as intermolecular forces and molecular packing. Therefore, it is not a reliable method for calculating the molecular weight.

Freezing point:
The freezing point of a substance is the temperature at which it changes from a liquid to a solid state. Like density, the freezing point is influenced by various factors such as intermolecular forces and molecular size. While the freezing point can give some indication of the molecular weight, it is not a direct measurement and can be affected by impurities or other factors.

Vapour pressure:
Vapour pressure is the pressure exerted by the vapour of a substance in equilibrium with its liquid or solid state at a given temperature. Vapour pressure is directly related to the molecular weight of the substance. According to the ideal gas law, the vapour pressure of a substance is inversely proportional to its molecular weight. Therefore, by measuring the vapour pressure and knowing the temperature, the molecular weight of a substance can be calculated accurately.

Vapour density:
Vapour density refers to the ratio of the density of a gas to the density of hydrogen gas at the same temperature and pressure. It is a useful parameter for determining the molecular weight of a substance. By measuring the vapour density, the molecular weight of a substance can be calculated using the formula:

Molecular weight = 2 x Vapour density

Since the question specifically mentions that the correct answer is option 'B' (Vapour density), it indicates that vapour density is the most appropriate method for calculating the molecular weight of a substance.