I'd question the source here a little bit. The singular "that" in A and B is off - do we know that there's only one other study on earlier works on slavery? It's much, much more logical that we'd need "those" here to show that Blassingame's study is unique among many. To use "that", the sentence would need to better denote that we're comparing Blassingame's study to one other...you'd really need to mention it by name "Unlike that of Wilson, Blassingame's study...".
Even with that, you don't need to compare "study" directly to "study". "Earlier works" are similar enough to "study" that it's an apt comparison. Each term can refer to an academic undertaking ("Einstein's early works on relativity are quite similar to Edison's first study on electricity in that...), so there's no fatal flaw in choice D. D is about as good an answer choice as you'll find for this question, so if you picked D I'd say move on confidently - you're right!
A... I guess they may be trying to equate these two things:
1) Studies (or the general term "study" not for one particular study, but rather "the study of physics..") of early writings on slavery
2) Blassingame's study of other, later biographies and memoirs on slavery
BUT this sentence doesn't go far enough to mandate that that is the ONLY potential apt comparison that can be made. You're not at all wrong for comparing:
A) Blassingame's study on slavery
B) Earlier works on slavery (that came before Blassingame's more recent study, the first of its kind)
So, like I said, I don't think this could be an official GMAT question - the author admirably tried to add that great GMAT-type subtlety but didn't close off the option of other apt comparisons like an official question (they cost thousands of dollars to write and test, so they're amazingly airtight) will. On this one, D is a pretty clear correct answer without much doubt; if you follow that logic I threw out there A may have some merit but it's a bit roundabout and you still can't eliminate A.
I'd mark this question as interesting practice but not much for a test-taker to sweat...if you've read this far in this thread you've more than learned enough from this one!
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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