---we have learned in school is beneficial to our future car

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