楼上答案有误,请楼主斟酌!
这道题应该选择D
解答这道题的关键在于manage to do sth(成功做某事) 和 理解 with his friend's assistance（在他朋友的帮助下）,这个短语的功能相当于一个插入语,可以移到句尾,再看整个句子的意思.
Although it was very late when John began doing the job,he managed to finish before the building was locked with his friends assistance.
做了如上变换后,可以看出,这道题考察的语法点,实际上是 manage to do sth.成功做了某事,所以选择D
整句话的意思是：尽管John开始做这项工作时已经很晚了,但是在朋友的帮助下,他成功地在写字楼锁门之前完成了工作.