Final answer:
Density is the property used to determine if an object is made of pure silver, as it is a physical property that can be measured without changing the substance's identity.
Step-by-step explanation:
The property used to determine if an object is made of pure silver (Ag) is density. Density is a physical property of matter, which refers to the mass per unit volume of a substance and can be observed or measured without changing the substance's identity. Since pure silver has a known density, measuring the density of an object and comparing it with the density of pure silver can help determine the purity of the object. In contrast, conductivity is also a significant characteristic of silver, due to its high electrical conductivity, but it isn't as definitive as density in assessing purity, as other metals can also be conductive. Malleability and reactivity are properties that do not specifically differentiate pure silver from silver alloys or other materials.