Dear Lethal
I composed this passage as part of my tutorials for my online GMAT students. You will not find a source for these in any other place. Hence, you may take my answer and explanation as the official ones.
Now, you say:
Removing the parenthesis makes it "Unlike most volcanoes, the hot spots are harbored" and comparing most volcanoes to the hot spots doesn't seem correct to me
If you can not compare volcanoes with hot spots, what else is the comparison between? Volcanoes and earth’s crust? The intention is to compare volcanoes found along the boundaries with the hot spots found in the interiors.
(D) Hot spots, harbored on the Earth’s surface and lying deeply in the interior of the plates, unlike most volcanoes, are found along the boundaries of the continental and oceanic plates that comprise the Earth’s crust
In the case of Choice D, the parenthesis content is “harbored on the Earth’s surface and lying deeply in the interior of the plates, unlike most volcanoes” You must remove both the features i.e., “harbored on the earth’s surface” as well as “unlike most volcanoes” The you are left with –“Hot spots …… are found along the boundaries of the continental and oceanic plates that comprise the Earth’s crust”. Isn’t this an anti- thesis?
Therefore D is wrong
In most of the 700 plus questions, you find more of the altered intent, improper word usage examples with plenty of grey areas, making them quite time – consuming to solve in the pressure – cooker atmosphere of the real GMAT and leading to the downfall of many an aspirant, And that is the crux of GMAT
Hope this helps .