改错Mary is no longer that she was ten years ago.

在no longer后加上the one
原句是缺少了先行词,所以要补回去.因为关系词用了that,所以用one
用what也ok,因为what引导了表语从句what she was ten years ago.
通常what引导名词性从句(如宾语,主语,表语从句等)的时候,既充当从句的一个句子成分,又充当主句的句子成分.譬如,现在的句子中,what充当了主句Mary is no longer (what) 的表语,在从句中,它也充当了表语:she was (what) ten years ago