选C的话用the one 可以,这样整个句子就是by learning a language different from the one which their parents speak at home.
不能用 one 因为one 起不到先行词的作用 而 the one 却可以/
“用that代替上句提到的词”是定语从句,应该是初三的内容吧,不知道lz的初中老师有没有系统讲过.该句型的结构是 先行词+that/which等关系代词+从句
“用one代替上句提到的词”是上句中已提到某一类事物,此句中想单独说一个.例句:“The apples are nice.I want to have one.”
“用the one代替上句提到的词”即是定语从句中的先行词.