1.你可以把它理解成是一个定语从句,the town hall (which was)completed in 1800's.B是非谓语动词,一般放在句首,逻辑主语和句子主语应该一致.
如果这个句子是 Having been completed in the 1800's,the Town Hall was the most distinguished building at that time.也对.
2.girl 和 受教育之间是一个被动关系,所以选B.
3._______so much in the war has made him more thoughtful.首先得搞清楚句子的主语是什么,很显然是made之前的他在战争中的所见.选A.Having seen 首先它是一个主动的,因为没有been,所以他的主语是人.而这个句子的主语不是.