Replace the phrase inside the quotation marks:
Construction of the Roman Colosseum, $\textit{"which was officially known as the Flavian Amphitheater, began in A.D.69, during the reign of Vespasian," }$ was completed a decade later, during the reign of Titus, who opened the Colosseum with a one-hundred-day cycle of religious pageants, gladiatorial games, and spectacles.
- which was officially known as the Flavian Amphitheater, began in A.D. $69$, during the reign of Vespasian,
- officially known as the Flavian Amphitheater, begun in A.D. $69$, during the reign of Vespasian, and
- which was officially known as the Flavian Amphitheater, began in A.D. $69$, during the reign of Vespasian, and
- officially known as the Flavian Amphitheater and begun in A.D. $69$, during the reign of Vespasian it