Final answer:
The property that classifies an unknown substance as a pure substance is being composed of only one element.
Step-by-step explanation:
The property that would allow an unknown substance to be classified as a pure substance is d) The unknown substance is composed throughout of one and only one element. Elements are pure substances because they consist of only one type of atom and cannot be broken down into chemically simpler components. This characteristic ensures a constant composition throughout the sample, a defining feature of a pure substance. Additionally, electrolytes and nonelectrolytes can be differentiated by their ability to conduct electricity. Electrolytes contain dissociated ions in solution that enable them to conduct an electric current, whereas nonelectrolytes do not conduct electricity when dissolved in water due to the lack of free ions.