They see you as something of a worrier,problem___which don't exits and crossing bridges long

这个从句有点问题吧？which don't （后面肯定是动词原形的啊）exits（不是原形） and（连接两个并列成分） crossing（和exit又不并列） bridges long before you come to them
你再看看原题，它就是这个样子吗？

这就明白点了，我觉得应该选B，exist是存在的意思，要是把整个句子翻译出来就是
They see you as something of a worrier,problem___which don't exist and crossing bridges long before you come to them.他们认为你总是能发现麻烦事，甚至连那些不存在的，或是沉寂很久的麻烦，你都能找出来

我是这么理解的setting是制造麻烦，但这个crossing bridges long before（暗示已经存在的意思吧）有点不搭，而see做后置定语很怪，一般我们不说设计（design）麻烦吧，所以我选B，只是我的见解，但不是肯定的哦~~

选C：还有……是seeing problems which...你打错了:D

see的基本意思是“看见”。在此是引申义“设想、想象”（=imagine）“把…看作…”的意思。他们把你看作爱担忧的人，总设想并不存在的问题，真是杞人忧天。此处crossing bridges before you come to them 的意思是wasting time thinking about difficulties which may never happen。

the whole sentence is not making much sense because where you put the space....

but i'm guessing it's C.
though none of them make much sense

横线和problem的顺序错了，应该换一下，______ problems which don't exist,
然后选C