The Tang dynasty was an imperial dynasty of China preceded by the Sui Dynasty and followed by the Five Dynasties and Ten Kingdoms period. It ended drastically when the empress Wu Zhao from the second Zhou Dynasty usurped the throne.
The Tang Dynasty had it´s capital in Chang`an, actually Xi`an, on that time it was the most populated city of the world, the period on which they reign is considered by some historians as a moment of splendor in Chinese civilization.
When the Han Dynasty fell on the year 220 A.C. China lived a period of social mayhem, politic disorder, economic and cultural isolation between the south and the north. On the year 589 A.C. the country was reunified by the Sui Dynasty and began a time of prosperity untill the arrival of the emperor Yang Di who failed on conquering the northern part of the Korean Peninsula. In front of this the Duke of Tang Li Yuan and his son and daughter, Taizong and Pingyang, rosed in rebelion in 617 and relegated the emperor into the position of retired emperor and acted as regent of the child emperor Yang You. But after Yang´s death in the year 618 Li Yuan proclamed himself as the emperor of a new dynasty, the Tang.
Therefore he was known as emperor Gaozu and ruled untill 626 when he was forcelly deposed by his son Taizong who have killed two of his brother due to fear of assassination, shortly after that his father left he throne to him.
Here is a briefly resume of the early begginings of the Tang Dynasty, from that we can notice that the first ruler of the Dynasty is not among the options given in the question, don´t confuse Gaozu with Gaozong who was the third emperor of the Dynasty. But making a elimination process of the options given we must dismissed Wu Zhao and Taizu because they are from differents Dynasties so the only option available is Taizong, the second emperor of the Dynasty.
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