Adding a salt to water leads to increase in the boiling point of solution with respect to water because:

Vapour pressure of solution is decreased

The correct answer is option 4. Vapour pressure of solution is decreased.

When a non-volatile solute, such as salt, is added to a solvent like water, it affects the colligative properties of the solution. One of these properties is the elevation of the boiling point. The correct explanation for the increase in the boiling point is related to the reduction in vapor pressure.

Boiling Point Elevation:
Adding a non-volatile solute, like salt, to a solvent (water) causes an increase in the boiling point of the solution compared to the pure solvent. This phenomenon is known as boiling point elevation.

Vapor Pressure:
The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. When a solute is added to a solvent, it interferes with the ability of the solvent molecules to escape from the liquid phase and enter the vapor phase. As a result, the vapor pressure of the solution is lower than that of the pure solvent.

Raoult's Law:
Raoult's Law describes the vapor pressure of an ideal solution. According to Raoult's Law, the vapor pressure of a solution is directly proportional to the mole fraction of each component in the solution.

Vapor Pressure Reduction:
When a solute is added to a solvent, the vapor pressure of the solution is lower than that of the pure solvent. This reduction in vapor pressure results in an increase in the boiling point of the solution. The solvent molecules now need to be heated to a higher temperature to reach a vapor pressure that matches the atmospheric pressure and allows the liquid to boil.

Therefore, the correct answer is (4) Vapour pressure of the solution is decreased.