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Hi, there. I'm happy to help with this.

First of all, the pronoun "each" is always singular, so the answers that pair "each" (singular) with "their" (plural) are incorrect. That immediately eliminates (A), (B), and (E).

The problem with (C) is ---- the relative pronoun "which" is supposed to introduce a relative clause, with it's own noun + verb. Instead, in (C), "which" is followed by a participial phrase, "contributing . . ." That's grammatically incorrect. It's also unnecessarily wordy.

That leaves (D) ---- compact, direct, and free of errors ---- the correct answer.

Does all this make sense? Please let me know if you have any more questions.

Mike

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Magoosh GMAT Instructor
http://gmat.magoosh.com/

Dear fantray,

Why does "which" need to be followed by a noun + verb? Well, how well do you understand independent and dependent clauses? Unlike a participial phrase ("contributing according to its ..."), any clause, whether independent or dependent, need to have a noun & verb. Dependent clauses are introduced by subordinate conjunctions (e.g. after, although, as, as far as, as if, as long as, as soon as, as though, because, before, if, in order that, since, so, so that, than, though, unless, until, when, whenever, where, whereas, wherever, and while) or a relative pronoun (e.g. who, what, which, where, when, why). These are huge topics in grammar --- I don't think there's any way I could say everything there is to say about them in a single post. Here's a free lesson on SC

http://gmat.magoosh.com/lessons/600-general-sc-strategies

but I would recommend some source that would give you a thorough overview of the grammar on the GMAT. Magoosh has an excellent series of videos covering all this grammar. See the link in my signature.

As far as what would make C acceptable . . . here is the current version of (C)

(C) In his experiments with gravity, Isaac Newton showed how the motion of each planet in the solar system results from the combined gravitational pull of the Sun and all the other planets, each of which contributing according to its mass and distance from others.

Here's the corrected version --- change the participle to a verb -----

(C) In his experiments with gravity, Isaac Newton showed how the motion of each planet in the solar system results from the combined gravitational pull of the Sun and all the other planets, each of which contributes according to its mass and distance from others..

The relative pronoun "which" acts as the noun and "contributes" is a bonafide verb.

Does that make sense?

Let me know if you have any further questions.

Mike

_________________
Magoosh GMAT Instructor
http://gmat.magoosh.com/

The construction ...
each of which contributes according to its mass and distance from others.
... uses the regular present tense, which is perfectly correct.

The construction ...
each of which is contributing according to its mass and distance from others.
... use the present progressive. The present progressive emphasizes that the action is question is taking place simultaneously with our description of it. "She talks to her friend on the phone" --- general present tense indicates merely that this is a regular occurrence, a typical recurring situation. "She is talking to her friend on the phone" --- the present progressive indicates that right now, right at this moment, she is performing this action, simultaneous with our description of it.
In this context, the planets are out their doing their gravitational thing, but there's absolutely no reason to emphasize that their action is exactly simultaneous with our description of it. That's why the present progressive would be incorrect. Yes, in the very narrow sense of providing a noun for the clause following "which" --- in that limited sense, it is correct, but overall, it does not fit the logic of the sentence.

Does that make sense?

Mike

_________________
Magoosh GMAT Instructor
http://gmat.magoosh.com/

That's a great question, gmatdriller. I'm happy to answer.

Let's go back to the original sentence with correct answer (D)
In his experiments with gravity, Isaac Newton showed how the motion of each planet in the solar system results from the combined gravitational pull of the Sun and all the other planets, each contributing according to its mass and distance from others.

You propose an alternate to (C):
In his experiments with gravity, Isaac Newton showed how the motion of each planet in the solar system results from the combined gravitational pull of the Sun and all the other planets, all contributing according to their mass and distance from others.

Yes, now we have agreement between "all" and "their", so that's no longer a problem. One problem that remains is: "their" refers to all the planets, but "mass and distance" are singular. Not all th planets have the same mass. Each has a unique mass. Therefore, we would need those words in the plural.
In his experiments with gravity, Isaac Newton showed how the motion of each planet in the solar system results from the combined gravitational pull of the Sun and all the other planets, all contributing according to their masses and distances from others.

This version is technically grammatically correct, insofar as all the singulars & plurals match up correctly. The problem is more subtle, about the logic of the sentence. We want to emphasize the complexity of Newton's model --- the motion of any one planet depends on the Sun's gravity and on the gravity from each other planet. Say we are looking at Mars and its orbit --- the Sun's gravity has one impact, Earth's gravity on Mars has another impact, Jupiter's gravity on Mars has yet another impact, etc. etc. The orbit of Mars depends on all the different impacts. Each planet has a unique impact on each other planet: that is the nature of the complexity of the Newtonian model.
The construction "each contributing according to its" captures the particularity of the interactions, whereas "all contributing according to their" lumps everything together in way that might obscure the full complexity. Therefore, the "each" version is better than the "all" version. BTW, this final distinction is, in all likelihood, more subtle that the GMAT SC would address.

Does that make sense? Let me know if you have any further questions.

Mike

_________________
Magoosh GMAT Instructor
http://gmat.magoosh.com/