What measures the proportion of a given area used for residential purposes?

The correct answer focuses on the concept of density, which refers to the number of residential units or the extent of land used for residential purposes within a specific area. Density is a crucial measure in urban planning and real estate, as it helps to assess how much space is allocated for housing compared to other uses such as commercial or industrial purposes.

When urban planners or developers evaluate a given area, understanding its residential density can guide decisions regarding zoning, infrastructure, resource allocations, and community development. Higher density may indicate a more urbanized area with more housing opportunities, while lower density might suggest a suburban or rural setting.

The other options do have applications in real estate and planning, but they do not measure the proportion of an area used specifically for residential purposes. Demand relates to market interest in housing or real estate, depreciation refers to the decrease in property value over time, and a deed of trust is a legal document related to financing property, not a measure of land use.