Explanation:

The vapour pressure of a solution is dependent on the concentration of solute particles in the solution. According to Raoult's law, the vapour pressure of a solution is equal to the product of the mole fraction of the solvent and the vapour pressure of the pure solvent.

In this case, the solute is Na2S, and it dissociates completely in water to form Na+ and S2- ions. Therefore, the concentration of solute particles in the solution is equal to the concentration of Na+ ions.

The question asks which of the given changes will cause the vapour pressure of the solution to increase. Let's consider each option:

a) Addition of 0.05 molal Na2S:
If we add 0.05 molal Na2S, it means we are increasing the concentration of Na+ ions in the solution. This increase in concentration of solute particles will decrease the mole fraction of the solvent, resulting in a decrease in the vapour pressure of the solution. Therefore, this option will not cause the vapour pressure to increase.

b) Addition of 0.5 molal Na2S:
If we add 0.5 molal Na2S, it means we are adding the same concentration of solute particles that is already present in the solution. This will not change the concentration of solute particles, and therefore, it will not change the vapour pressure of the solution.

c) Addition of 2 molal Na2S:
If we add 2 molal Na2S, it means we are increasing the concentration of solute particles in the solution. This increase in concentration of solute particles will decrease the mole fraction of the solvent, resulting in a decrease in the vapour pressure of the solution. Therefore, this option will not cause the vapour pressure to increase.

d) Addition of 5 molal Na2S:
If we add 5 molal Na2S, it means we are increasing the concentration of solute particles in the solution. This increase in concentration of solute particles will decrease the mole fraction of the solvent, resulting in a decrease in the vapour pressure of the solution. Therefore, this option will not cause the vapour pressure to increase.

Conclusion:
From the above analysis, we can conclude that the only option that will cause the vapour pressure of the solution to increase is option 'A' - Addition of 0.05 molal Na2S.