Final answer:
A pure substance has a uniform composition, as it consists of only one type of matter throughout, which can be an element or a compound.
Step-by-step explanation:
A pure substance is a form of matter that has a constant composition and properties that are constant throughout the sample. This means that it is the same everywhere and exhibits a uniform composition from point to point. Pure substances include elements, which consist of only one type of atom, and compounds, which consist of two or more types of atoms bonded together. A pure substance is not the same as a mixture, which may have variable composition and properties.
The correct characteristic of a pure substance is A) uniform composition.