The unique element of the Arch of Constantine is that it reused sculptures from earlier monuments, known as spolia. This practice connects Constantine’s reign to the broader history of Rome while highlighting its architectural significance. Therefore, the correct answer is B: It used previously existing sculptures from other monuments.
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The Arch of Constantine is unique primarily because it reused sculptures from earlier monuments, a practice known as spolia. This integration connects Constantine's reign to Rome's historical legacy while highlighting the arch's significance in Roman architecture. Other mentioned aspects do not represent this uniqueness as clearly. ;