Have you ever asked him the reason_____ may explain his absence?A.why B.when C.that D.what为什么选C?怎么不选D呢?

【C】
此题为定语从句，the reason为先行词，从句may explain his absence缺主句，所以需要关系代词that来充当主语，

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

C
_____ may explain his absence为定语从句，修饰the reason .定语从句缺主语，故选C.

正确答案原因上面说的很清楚,我写几个自己改的句子吧
如果用what,句子就不是这样了,后面一半要加具体的物而不是单纯引导定语从句了
Have you ever asked him to explain what's the reason that he's absent?
用why
Have you ever asked him the reason why he's absent?
when在这里实在不靠谱,就不谈了
可以看出其实句子到have you asked him the reason结构已经完整了,后面只是补充说明说法很多,但是是由具体语序导致的成分变化决定的答案
希望能给你帮助

这是定语从句，the reason 在从句中做主语