In unveiling vast swaths of areas of which they previously had faintly

Dear Samcom,

I'm happy to respond. This question was written by my brilliant colleague & friend, Chris Lele. It's a tricky comparison.

A. In unveiling vast swaths of areas of which they previously had faintly any notion of their existence, much like modern day astronomers, ancient cartographers
The phrase "of which they previously had faintly any notion of their existence" is profoundly awkward and indirect. This is a very clumsy way to put these ideas together. Choice (A) is incorrect.

B. Much as modern day astronomers do, ancient cartographers unveiled vast swaths of areas of which they faintly had any notion, thereby
Sleek, direct, concise. No obvious grammar or logic problem. This is promising.

C. Like modern day astronomers, ancient cartographers unveiled vast swaths of areas whose previous existence they faintly had any notion of,
Hmm. Notice that (A) & (B) framed the analogy loosely: we can make an argument for the similarity of these people in very different time periods. Choice (C) chucks that subtlety and states it baldly: "Like modern day astronomers, ancient cartographers . . ." This is a subtle change in meaning, because the analogy is stated more forcefully, without hedging. That's a big problem with (C). Also, notice that the phrase ends with a preposition: that's a telltale sign. Ending with a preposition will NEVER be a deciding split, but notice that it only every appears on wrong answers on the GMAT SC. Choice (C) is incorrect.

D. Ancient cartographers, much like modern day astronomers, unveiled vast swaths of areas, the existence of which they faintly had any notion of previously, thereby
The phrase "the existence of which they faintly had any notion of previously" is another clunker, awkward, indirect, and clumsy. Choice (D) is incorrect.

E. Much as modern day astronomers have, ancient cartographers unveiled vast swaths of areas the existence of which they previously had faintly any notion,
This isn't bad, but it's wordier than (B). Notice that all the answer choices use "existence" but (B) doesn't. Use of this word is redundant. Consider these two sentences:
(a) In 1898, physicists became aware of the electron.
(b) In 1898, physicists became aware of the existence of the electron.
Those two sentences have the exact same meaning, but (b) uses more words to say the same thing. In other words (b) is redundant. Much in the same way, even (E) is redundant. The word "previously" is also redundant, because if discover something new, it's obvious that previously I didn't know it. The GMAT has zero tolerance for redundancy. Consider these two versions:
(E) Much as modern day astronomers have, ancient cartographers unveiled vast swaths of areas the existence of which they previously had faintly any notion,
(E1) Much as modern day astronomers have, ancient cartographers unveiled vast swaths of areas of which they had faintly any notion,
Now, (E1) looks good: in fact, it's quite similar to (B). When we remove all the redundancies, we get a sleek direct sentence.

The best answer here, the only possible answer, is (B).

Here are four more practice questions of this ilk:
4 Challenging Comparison Questions on the GMAT

Does all this make sense?
Mike