选2,what
整个句子为主、系、表结构：An opinion /is /what someone believes is true but has not been proved.
这个句子比较复杂的是表语,是一个表语从句（也属于名词性从句）.因为从句的宾语语中缺少成分而是用"what"引导.
如果将WHAT放回从句中,从句正常的语序是：someone believes/ "what" is true but has not been proved.从句谓语是believes,后面是一个宾语从句,含有两个并列句,但主语都是what,因此but后面省略了主语.（what)has not been proved是一个被动句.
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