The correct answer is A. A homogeneous mixture of two or more substances.
A solution is a homogeneous mixture of two or more substances. The solute is the substance that is dissolved in the solvent. The solvent is the substance that does the dissolving. In a solution, the solute and solvent are evenly mixed together and cannot be separated by physical means.
Option B is incorrect because a solid dissolved in a liquid is not necessarily a solution. For example, a suspension is a heterogeneous mixture of a solid and a liquid. The solid particles in a suspension are large enough to be visible to the naked eye and will eventually settle out of the mixture.
Option C is incorrect because a solid dissolved in water is not necessarily a solution. For example, a saturated solution is a solution that contains the maximum amount of solute that can be dissolved in a given amount of solvent at a given temperature. If more solute is added to a saturated solution, the excess solute will not dissolve and will form a precipitate.
Option D is incorrect because a mixture of two liquids is not necessarily a solution. For example, an emulsion is a heterogeneous mixture of two or more liquids that do not mix together. The liquids in an emulsion are immiscible, which means that they do not dissolve in each other.