Hello Everyone!
Let's tackle this question, one problem at a time, and narrow it down to the correct answer! Before we dive in, let's take a look at the original question, and highlight any major differences between the options in orange:
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.
(A) Adam Smith's two major books are to democratic capitalism what
(B) Adam Smith's two major books are to democratic capitalism like
(C) Adam Smith's two major books are to democratic capitalism just as
(D) Adam Smith wrote two major books that are to democratic capitalism similar to
(E) Adam Smith wrote two major books that are to democratic capitalism what
After a quick glance over the options, there are 2 main things we can focus on:
1. Adam Smith's two major books / Adam Smith (modifier-antecedent agreement)
2. what / like / just as / similar to / what (idiom structure; comparisons)
Let's start with #1 on our list because it will eliminate 2-3 options rather quickly. If we look at the entire sentence, we see that it begins with a modifier:
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.
Who/what was a leading figure in the Scottish enlightenment? Adam Smith, or Adam Smith's books? It makes the most sense for it to be Adam Smith, right? That means whatever comes after the modifier needs to clearly show that it's referring to Adam Smith only:
(A) Adam Smith's two major books are to democratic capitalism what
(B) Adam Smith's two major books are to democratic capitalism like
(C) Adam Smith's two major books are to democratic capitalism just as
(D) Adam Smith wrote two major books that are to democratic capitalism similar to
(E) Adam Smith wrote two major books that are to democratic capitalism what
We can easily eliminate options A, B, & C because the modifier is trying to refer to "Adam Smith's two major books" instead of just Adam Smith himself.
Now that we only have 2 options left, let's focus on #2 on our list: idiom structure with comparisons! To make this easier to spot, I added the ending of the original sentence to round out the comparison:
(D) Adam Smith wrote two major books that are to democratic capitalism similar to Marx's Das Kapital is to socialism.
This is INCORRECT because it doesn't follow the correct idiom structure for comparing items:
INCORRECT = X is to Y similar to A is to B
(E) Adam Smith wrote two major books that are to democratic capitalism what Marx's Das Kapital is to socialism.
This is CORRECT because it uses the proper idiom structure for comparing items:
CORRECT = X is to Y what A is to B
There you have it - option E is the correct choice!
Don't study for the GMAT. Train for it.