Which of the following best describes a solution?

A solution is best described as a homogeneous mixture of two or more substances. In a solution, the components are uniformly distributed at a molecular level, meaning that the composition is consistent throughout. This characteristic distinguishes solutions from other types of mixtures, such as heterogeneous mixtures, where the individual substances can be visibly distinguished and do not blend uniformly.

For example, when salt dissolves in water, it forms a solution where the salt is evenly distributed throughout the water, resulting in a single-phase liquid. This level of uniformity is what defines a solution and sets it apart from mixtures that may involve solid particles or phases that do not combine at a molecular level.

Other types of mixtures and reactions described in the options do not fit the definition of a solution. Mixtures of solids only, like a mixture of sand and gravel, can be heterogeneous and are not considered solutions. A type of chemical reaction does not describe a physical mixture but rather a process where substances transform into new substances, and mixtures that can only be solid do not meet the criteria of a solution since solutions can also be liquids or gases. Thus, the option defining a solution as a homogeneous mixture of two or more substances accurately captures its essence in chemistry.