Help！再问两道SC，完全不懂啊

Q29:

A leading figure in the Scottish enlightenment, Adam Smith’s two major books are to democratic capitalism what Marx’s Das Kapital is to socialism.

1. Adam Smith’s two major books are to democratic capitalism what
2. Adam Smith’s two major books are to democratic capitalism like
3. Adam Smith’s two major books are to democratic capitalism just as
4. Adam Smith wrote two major books that are to democratic capitalism similar to
5. Adam Smith wrote two major books that are to democratic capitalism what

答案E

最后那个what是什么意思？什么叫what Marx’s Das Kapital is to socialism.???