Which of the following describes pure substances?

Pure substances are defined by having consistent and uniform properties throughout. This means that they possess definite chemical and physical properties, such as a specific melting point, boiling point, and appearance. These characteristics remain unchanged regardless of the sample size or conditions, distinguishing pure substances from mixtures, which can vary in composition.

Pure substances can be elements, which consist of a single type of atom, or compounds, which are made up of two or more types of atoms that are chemically bonded together in a fixed ratio. This consistent composition is what allows them to have specific chemical behavior and characteristics, making option C accurate in describing pure substances.

The other choices do not accurately represent the nature of pure substances. For instance, mixtures involve combinations of pure substances and do not maintain consistent properties. Indefinite chemical properties and the presence of solvents pertain to mixtures and solutions rather than pure substances. Thus, option C is the only statement that correctly encapsulates what pure substances are.