Construction of the Roman Colosseum, 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.

A. which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian,

B. officially known as the Flavian Amphitheater, begun in A.D. 69, during the reign of Vespasian, and

C. which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian, and

D. officially known as the Flavian Amphitheater and begun in A.D. 69, during the reign of Vespasian it

E. officially known as the Flavian Amphitheater, which was begun in A.D. 69, during the reign of Vespasian, and

OA is C,

my doubt is why is b incorrect?

A) cant we use begun here? or should begun be preceeded by "had"?
B) why is "which" needed here ?