she want to know the reason______fewer and fewer students are showing interest in her lesson

选B
解析：在这个长句中,the reason是一个先行词,后面的部分是定语从句,于是选择关系副词why .she want to know the reason这是个完整的句子的,why fewer and fewer students are showing interest in her lesson 是为了修饰这个“原因”那for 为什么不可以前面是the reason 表示“原因”，而for也可以表示原因，两者一般不同时出现在句子里。例如，可以说：She is a good teacher for her kind heart .在这里可以用for或者用because of 也可以。