Which of the following is NOT a common design feature of a Roman forum?

The presence of a garden is not a common design feature of a Roman forum. Roman forums were primarily public spaces that served as the center of civic life, and their design was heavily influenced by Roman architectural styles and functionality. Common elements found in a Roman forum include temples, which were places of worship and often dedicated to various gods; courtyards, which provided open space for gatherings and public meetings; and colonnades, which were rows of columns that framed and defined the space, enhancing both aesthetics and structural integrity.

Gardens, while present in some Roman architecture, were usually associated with private villas or estates rather than public forums. A garden would not typically serve the civic and political purposes of a forum, making it distinct from the other features that are integral to the function and design of these public spaces. Thus, the correct answer identifies a feature that does not align with the traditional understanding of a Roman forum's layout and purpose.