There is only one thing( ) I can do.A.what B.that C.all D.wh

这个选C
解析：语法有规定,当前面语句中出现 “only”时,后用从句必须用that作连词或者省略不填.
翻译：这是我唯一能做的事情.
相关语法补充：
当先行词之前有the only, the same, the very, the first, the last或all, only, any, no, every, some, much等形容词、序数词修饰的时候,定语从句要用that来引导,如：
Man is the only creature that is gifted with speech.
人类是唯一有语言天赋的动物.
He was the first man that came.
他是最先来的人.
Any paper that you read will give the same story.
你看到的任何报纸都会报道同样的新闻.