D.
原因：英语中“因为”“所以”通常只用一个.有because,since就不用so.
It 为虚拟主语,实际主语为“That he succeeded in the end" (记得加”that“哦,否则就不是句子成分,而成了独立的句子了）.去掉虚拟主语的话,句子为：That he succeeded in the end was because he worked hard and persistently.
A.Since he worked hard and persistently,he succeeded in the end.
B.Because he worked hard and persistently,he succeeded in the end./
He worked hard and persistently,so he succeeded in the end.(不太常见这么说的）
C.As he worked hard and persistently,he succeeded in the end.