What is a solution?

A solution is defined as a homogeneous mixture where one substance, known as the solute, is dissolved in another substance, called the solvent. In most common contexts, such as in chemistry, when referring to solutions, we often deal with liquid solutions, where a solid, liquid, or gas is dissolved in a liquid medium. For example, when table salt is dissolved in water, the salt is the solute and the water is the solvent, resulting in a saline solution.

In contrast, options that describe a gas mixture, a combination of solids, or a solid with varying densities do not align with the definition of a solution. Gas mixtures consist of different gases that may not interact or dissolve in one another, while a combination of solids lacks the uniformity associated with solutions. Similarly, solids with varying densities do not result in a homogenous mixture characteristic of a solution. Hence, the concept of a solution specifically pertains to liquids with dissolved substances, making the definition provided in the correct answer accurate and essential for understanding what constitutes a solution in the realm of chemistry and water quality.