The fundamental economic principle used in property valuations that refers to the ability to render or service a product that satisfies demand is:

The correct answer is utility. In property valuations, the concept of utility refers to the ability of a property to meet the needs of consumers or satisfy a particular demand. When a property has utility, it means that it offers features or benefits that are valuable to potential buyers or tenants, enhancing its desirability and overall value.

Utility is a critical factor because it directly correlates to how well a property can fulfill the requirements of its users. For example, a property with functional layouts, convenient locations, and amenities that meet the needs of occupants will have greater utility, thereby increasing its market value.

The other concepts, while related to economic principles, do not specifically address the ability to serve demand in the context of property values. Scarcity pertains to the limited supply of property relative to demand but doesn’t capture how well a property can satisfy that demand. Desire focuses more on consumers' wants and preferences rather than the inherent value derived from utility. Purchasing power relates to the financial means of buyers to acquire property but does not influence the property’s ability to fulfill needs. Therefore, utility is the most relevant concept when determining property valuations based on its capacity to serve demand effectively.