Which of the following best describes a setback requirement in zoning?

A setback requirement in zoning refers to the specified distance that a structure must maintain from a property line. This regulation is put in place to ensure adequate spacing between buildings, to promote safety, privacy, and aesthetic considerations in a neighborhood. By controlling how far buildings can be from the boundaries of a property, zoning laws help manage density, provide access to light and air, and reduce the potential for conflicts between neighboring properties.

The other options address different aspects of zoning regulations. For example, limits on the height of buildings focus on controlling how tall structures can be in a given area, which is not related to the distance from property lines. Requirements for green spaces pertain to landscaping and open areas but do not define the spatial relationships of buildings to property boundaries. Rules about building materials concern the standard and type of construction used, which again does not involve spatial distance relative to property lines. Therefore, the specificity of setback regulations directly aligns with the description provided in the correct answer.