Which of the following describes substances that include a uniform composition?

A substance with a uniform composition is defined as having consistent characteristics throughout. This means that every part of the substance has the same properties and composition, regardless of the sample size.

A solution is a specific type of mixture where one substance (the solute) is uniformly dissolved in another substance (the solvent). The result is a homogeneous mixture, which appears consistent throughout. For example, when table salt is dissolved in water, it forms a solution where salt ions are evenly distributed within the water, displaying the same concentration throughout.

Other types of substances like compounds and elements also reflect uniform composition. A compound consists of two or more elements chemically combined in fixed proportions, and it maintains a consistent composition throughout the substance as well. An element is a pure substance that cannot be broken down into simpler substances and also has a uniform composition.

However, while both compounds and elements are uniform, the term "solution" specifically conveys the context of a mixture where one component is dissolved into another, emphasizing the uniform distribution of particles in that scenario. Hence, the choice that best describes substances with a uniform composition in the context provided is solution.