What innovative feature does the Pantheon showcase that was unprecedented in its time?

The Pantheon is renowned for its enormous dome, which was a groundbreaking engineering achievement of its time. This dome spans 43.3 meters in diameter and is constructed from concrete, showcasing the advanced Roman engineering techniques that allowed for such a large, open interior space without the need for supporting columns. At the center of the dome is the oculus, a circular opening that not only serves as a source of natural light but also reduces the weight of the structure at its pinnacle. The use of a dome combined with an oculus was unprecedented, marking a significant development in architectural design that influenced structures in the future, ultimately laying the foundation for Renaissance and modern dome construction. This innovative feature illustrates the Roman mastery of concrete and spatial design, highlighting the Pantheon's status as a monumental architectural achievement.

The other options, while related to various architectural elements, do not capture the uniqueness of the Pantheon. The large square base refers to many classical structures but does not define the exceptional engineering of the Pantheon. Stained glass, prominent in later Gothic architecture, was not a feature of ancient Roman buildings. Multiple levels of columns can be found in many classical designs but do not reflect the innovative dome that sets the Pantheon apart.