该题中"What we have learned in school"为主语从句,也就是说这一整个是在整个句子中充当主语.
如果你非得是要all作为主语,那么引导词应该用that,而不是what.当句子成为"All that we have learned in school is beneficial to our future career."时,"that we have learned in school"是定语从句.定语从句的引导词并不含有what.
因此,该题选A,而不是C