-Why did he say so?-sorry,I don't understand---he has said m

选C
很明显,此题考宾语从句和定语从句.
what只能引导名词性从句,不能引导定语从句,排除A.你需要掌握定语从句和名词性从句的引导词.
all也不能充当引导词,排除B和D.
再看C.understand和said都是及物动词,后面必须带宾语.而all放在这里非常适合：i don't understand all与he has said all都正确.而又加了一个定语从句,即“我不能理解的是他说的”,可见先行词是all,由于all充当先行词的定语从句必须由that引导（定语从句的特例）,所以综合看来,就是all that,选C
懂了吧?你需要把定语从句,宾语从句多看一下