Solubility is defined as the maximum amount of a specific solute that can dissolve in a given amount of solvent under specific conditions (temperature and pressure) to form a saturated solution. This intrinsic property determines the limit of how concentrated a solution can become.

The correct option is A - determines the amount of solute that can dissolve in a given amount of solvent.

B. lowers the vapour pressure of the liquid: The addition of a non-volatile solute to a solvent lowers the vapor pressure (a colligative property), but solubility itself is a measure of the amount that can dissolve, not the phenomenon of vapor pressure lowering.

C. enables us to know the chemical reactions of the gas with the liquid: Solubility describes a physical property (dissolution), not the chemical reactivity between a gas and a liquid.

D. raises the boiling point of liquid: The addition of a non-volatile solute raises the boiling point (another colligative property), but again, solubility is the measure of the extent of dissolution.