What walls are often shared by buildings on each side of a property line?

Party walls are the correct choice because they are specifically designed to be mutual structures that provide support for adjacent buildings located on either side of a property line. These walls serve a crucial function in urban settings, where buildings are often constructed in close proximity to one another. The design of party walls allows them to carry the vertical loads from both buildings, which helps in maintaining structural integrity and minimizing the amount of space required for construction.

In contrast, load-bearing walls are critical for a structure's integrity but are not necessarily positioned on property lines or shared between two buildings; they primarily support the weight of the roof and upper floors within a single building. Nonbearing walls serve to divide spaces within a building but do not support any structural load. Fire barrier walls, while they serve an important role in containing fires and limiting their spread between structures, do not specifically define the shared nature of party walls. Their purpose is more about providing a fire-resistant separation rather than the joint structural function found in party walls.