Have you asked her for the reason____may explain her absence

定语从句的格局是：名词 + 关系词 + 句子
如果名词在后面的句子中做某个句子成分,多半是主语、宾语、定语、状语或表语,如果reason that + 句子,那么reason 就是在后面句子中做主语、宾语.
例如：
the reason that was given by his mother 做主语
the reason that I told you 做宾语
如果 reason why + 句子,那么 reason 就是在后面句子中做状语,表示原因=for which
the reason why he was late = the reason for which he was late.
所以,选用定语从句前面关系词的关键是两点,一是前面的名词是什么内容,是人还是物,时间、地点还是原因的名词,二是判断前面的名词在后面的句子中做什么成分

可以