Which building served as the headquarters for the Chicago Fire Department before it was renovated?

The Old Main Post Office is the correct answer because it served as a crucial hub for the Chicago Fire Department prior to its renovation. Built in 1921, this historic building, located on the southwest side of downtown Chicago, was not only a functional post office but also housed various city departments, including the Fire Department. Over the years, it has undergone significant renovations to repurpose the space for modern use while retaining its historical significance.

The other buildings listed have distinct purposes and functions that do not relate to serving as the headquarters for the Chicago Fire Department. The Art Institute of Chicago is primarily an art museum, the Willis Tower is known for its commercial office space and as a tourist attraction, and the Chicago Cultural Center serves as a venue for cultural events and public gatherings. None of these buildings acted as headquarters for the Fire Department, emphasizing the unique role of the Old Main Post Office in Chicago's architectural and civic history.