What historic structure was built on the foundations of footraces and is known today as Piazza Navona?

The historic structure known today as Piazza Navona was indeed built on the foundations of the Stadium of Domitian, which was constructed in the first century AD. This stadium was primarily used for athletic contests, including footraces, and could accommodate thousands of spectators.

Over the centuries, as the use of the stadium declined, it was gradually covered and repurposed. In the 15th century, the site was transformed into the public square known as Piazza Navona, utilizing the original layout of the stadium. The oval shape of the square reflects the structure of the ancient stadium below, showcasing the continuity of urban space from antiquity through the Renaissance.

The other options reflect structures with different functions: a theater is typically associated with performances and not athletic events, a taberna refers to a type of shop or stall typically found in ancient marketplaces, and a tepidarium is a warm room in ancient Roman baths, also not related to athletic competition or the concept of a stadium. Thus, the correct identification of Piazza Navona as being built on the foundations of the Stadium of Domitian highlights its historical significance and architectural lineage.