1.B
which引导非限制性定语从句,代替整个主句He believes in himself,在从句___ is of...中充当主语.
in my opinion是该非限制性定语从句的插入语,可以忽略.
that不能用于引导非限制性定语从句,what根本都不能引导定语从句,排除；
as虽然也有此种用法,但是它一般翻译为“正如/就像...一样”,且用于被动句式较多,此处不如which准确.
2.C
同样是非限制性定语从句,且该从句采用了倒装语法.
即：where lives my uncle = in which lives my uncle = in Shanghai lives my uncle = my uncle lives in Shanghai
该定语从句缺少地点状语,只能使用关系副词where
3.A
此题考查强调句型和主语从句.
该句正常写法是：(That he often fails in exams) makes his parents worried about him.
该句的主语是一个主语从句,因为该从句不缺少成分,所以用that引导,但是that不能省略.
如果对该句的主语（从句）进行强调,即使用强调句型结构（It is/was + 被强调部分 + that + 句子未被强调部分）时,则直接变为：
It is + 被强调部分（that he often fails in exams）+ that + 句子未被强调部分（makes his parents worried about him）.
这就是答案A.