it is years of hard work that has made him what he is today.

原句是：
Years of hard work (主语） has made （谓语）him （宾语）what he is today（宾补）

此题是强调句, 强调的是原句的主语
强调句句型：
It is + 被强调部分+ that-从句
it is years of hard work that has made him what he is today


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再问：
再答： 知道名词可以做宾补么？ 如： We elected him monitor.(宾语和宾补有着逻辑上的主谓关系---He was monitor) what he is today这个就是名词性从句， 如果你学到一定境界， 它在你眼中就是一个“名词”， OK？ ：）