The correct answer is D. A pure substance
Step-by-step explanation:
In chemistry and related fields, a pure substance refers to a homogeneous substance in any state (solid, gas, liquid) that has defined and consistent chemical properties. This category includes chemical elements that have not reacted with other elements and compounds (mix of two or more elements) that are homogeneous.
In the case of oxygen that is represented by the symbol O this is a pure chemical element, this implies this substance has not been mixed with others, in it, all atoms are similar or identical, and it has defined chemical properties. Therefore, oxygen can be classified as a pure substance because it is homogeneous in composition and chemical properties (Option D).