Sentence Correction

Construction of the Roman Colosseum, which was offi cially 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 offi cially known as the Flavian Amphitheater, began in A.D. 69, during the reign
of Vespasian,

(B) offi cially known as the Flavian Amphitheater, begun in A.D. 69, during the reign of Vespasian,
and
(C) which was offi cially known as the Flavian Amphitheater, began in A.D. 69, during the reign
of Vespasian, and
(D) offi cially known as the Flavian Amphitheater and begun in A.D. 69, during the reign of Vespasian it
(E) offi cially known as the Flavian Amphitheater, which was begun in A.D. 69, during the reign of
Vespasian, and

OA C

I have a doubt with option B. The explanation given for B in OG is "Begun is the wrong verb form".

Will the choice B be correct if the verb was "began".
I reasoned it to be incorrect as I thought .the modifier officially known as the Flavian Amphitheater requires a which to clearly modify Colossem...
if the which is omitted, it might sound ...the construction was officially known as Flavian Amphitheater


Construction of the Roman Colosseum, officially known as the Flavian Amphitheater, began in A.D. 69, during

Please explain.. Am I correct