What term is used for public open space that must be free of obstructions affecting natural light and ventilation?

The correct term for public open space that must be free of obstructions affecting natural light and ventilation is an easement. An easement is a legal right to use another person's land for a specific purpose, which can include ensuring access to light and air. This type of public space is specifically designed to maintain certain public benefits, including the preservation of natural light and ventilation.

In urban planning and land use, easements are critical for protecting access to light and air, particularly in densely built environments where structures might otherwise block these essential elements from reaching adjoining properties. By establishing these rights, communities can promote healthier environments and improve overall living conditions.

The other options, while they pertain to types of public spaces, do not inherently cover the requirement for obstruction-free zones that specifically guarantee natural light and ventilation. Parks and greenways are designed for recreation and public enjoyment but do not have the same regulatory framework regarding obstructions. Commons generally refer to shared resources or spaces within a community without the specific focus on light and ventilation.