-----that is important is that you are doing your best and m

选B
缺成分,缺主语
含义：你正在尽全力朝着正确的方向努力才是最重要的!
句子主干是All is that+从句.主语是all,其作用有点像于代词,如同 it is a book中的it,只不过该句中all指代的是that引导的一个从句you are doing your best and moving in the right direction,从句在句中的成分是宾语.
that is important是all的定语从句.
all that is important is .是一种惯用结构,也不能说固定搭配,类似的结构还有,all that i can do is to tell you the truth,我所能做的就是告诉你真相(此处用的是不定时作宾语),all i want is a ipad.等等.