In my terminology
what is a relative pronoun which is inclusive of the antecedent in your sentence.
yaqi_wang“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.”
I consider this sentence ungrammatical. Adam Smith, who is clearly the intended leading figure, is in the genitive. Two major books cannot be the figure since books are not people, but that's what the sentence implies as two major books is in the nominative case - or the common case as many call it.
Alternative E makes the original sentence grammatical since Adam Smith isn't in the genitive. The use of similar in alternative D sounds unidiomatic to my ear, and, furthermore, similar is too vague in meaning for alternative D to correspond to the original sentence completely. Similar means "having a likeness or resemblance, especially in a general way".
CB