i think it should be C. because that implies only those element( Ur, Silver, mercury and iodine) which are stable and naturally occuring.

OE by KAPLAN
Read the Original Sentence Carefully, Looking for Errors:

The sentence contains no grammatical errors as written and has no glaring style issues. However, time permitting, the other answer choices should always be tested systematically, even when "correct as written" is expected.

Scan and Group the Answer Choices:

With no error to scan for and no obvious correct/incorrect split in the answer choices, Step 2 is impractical for this problem. Unfortunately, answers must be eliminated one by one.

Eliminate Wrong Answer Choices:

Choices (B) and (C) both reconstruct the underlined portion with more words and less clarity than the original. Eliminate them both.

Choice (D) introduces clarity problems by rearranging the original’s word order and placing the word "stable" right after the adjective "heaviest." Eliminate.

Choice (E) is very confusing, implying that uranium is a plural noun and making the sentence’s logic generally difficult to follow. Eliminate.

The sentence as written makes its point in a correct, concise, and stylistically sound manner, so Answer Choice (A) is correct.
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I reduced the choices to C and A and chose C. I felt that "that are stable and naturally occuring" is clearer than "of the stable, naturally occuring". I felt that if the comma in A after "stable", is replaced by "and" as in C it would have been perfect. Anyway, now we know there was no error in A.
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Let me try to help.

Noun: chemical elements
Adjectives: stable, naturally occurring
Generally, it's more common and concise to place the adjectives separated with commas before the Noun. At least, this observation has worked for me when stuck between choices such as A and C which are both grammatically correct and differ only in style.

Also, C misses the word "chemical" from the sentence altogether.

Example: Robert De Niro, the oldest among all the tall, well-built actors, is going to feature in the upcoming movie Last Vegas to be released before Christmas this year.

I will definitely follow your advice henceforth.
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No. It's quite common to separate two adjectives or adjective phrases with only a comma when they precede a noun, as in "The cow is a beautiful, intelligent animal." Now, if we use those adjectives predicatively, by putting them at the end after the verb, we do need "and": "This cow is beautiful and intelligent."
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Harshgmat , good instincts. This question is problematic. You opened a can of worms. Or handed me a can of worms to open?

I suspect that the authors believed C to be quite different from A.
I think the authors failed to draw the intended degree of distinction.

I reviewed all OG 2018 SC questions. Close calls are not as close as the one in this question.

In short, this question is not official and is probably too hard. I would not worry about it.

If we compare A and C directly it may be easier to notice that C subtly shifts emphasis away from the predicate in which the comparative abundance of uranium takes the spotlight.

In C, the modifier "of the elements that are stable and naturally occurring" and the predicate "more abundant than ..." have about equal weight.

(A) leans much more towards predicate emphasis.

In particular, C makes the incidental qualities of uranium (stable and naturally occurring) seem more important than A does.
Just before the verb in C is "naturally occurring." In A the predecessor is "elements," which directly references both uranium and the other three in the comparison.

(A) Uranium, the heaviest of the stable, naturally-occurring chemical elements, is more abundant in the earth's crust than silver, mercury, or iodine.

(C) Uranium, the heaviest of the elements that are stable and naturally occurring, is more abundant in the earth's crust than silver, mercury, or iodine.

The descriptors come before the noun elements in (A), rather than after as in (C). The adjectives both capture the other elements and allow uranium's comparative abundance to be the centerpiece.

In English, unless the author has deployed a different emphatic device, what comes at the end of the sentence gets the emphasis.*

In C, a lengthier noun description and "are" steal some of the heft from the thrust of the sentence as it is presented in A.

At the end of both C and A lie the other three elements. We likely can infer that the author intended to emphasize the comparative abundance of uranium.

One more tiny difference bears mention. (C) does not specify that the elements are "chemical." The other four options do. An element can be many things. We may know that uranium is a "chemical" element (or at least we know it is not, say, a "constituent part").

I suppose, however, for a person new to English, "chemical" would clarify "element." Perhaps "chemical" is weightier than it seems.

More to the point: the answer setup indicates that the author thinks that "chemical" matters.

I chose (A). Stylistically, it's a better sentence. In A, attributive adjectives that directly modify the noun are punchier than relative clause adjectives in C, in part because the linking verb "is" leads to another adjective.

Further, C's structure requires a that-clause with the verb ARE, which is a second instance of an inert and leaden "to be" verb.

(A) and (C) do not differ much in meaning. (A) is better because its word placement makes the sentence more engaging and forceful. That placement is an example of concision.

Although concision may be the least important of GMAT rules, this particular source often tests obscure issues.

Option A must seem to contain some error, I think. I cannot explain the stats below otherwise. Why not A over C?

I hope that helps!

Analysis of the question

Statistics for the answers are
Option A = 49%
Option C = 40%

The percentage of respondents who chose C is very high. I doubt that the authors thought that C would be so tempting.

Had they known, I hope that they would have given a better explanation than this assertion: "Choice[] (C) reconstruct[s] the underlined portion with more words and less clarity than the original." Why less clarity?

That sort of explanation mirrors too many OEs.

Because I was bothered, I mined data. In OG 2018, if the explanation of an option used only terms such as wordy, awkward, or confusing, I checked to see whether unmentioned but bigger errors existed.

23 options had explanations that were "style only." All 23 had other problems not listed in the OE (unidiomatic, not parallel, verb tense, pronoun errors, etc.).

Some options changed intended meaning very subtly, but none were as close in meaning to their corresponding correct answer as C is to A in this case.

This question's razor-edge decision point is likely too thin to qualify the question as sufficiently representative of a real GMAT question.

I intend no disrespect. These questions are very difficult to write. I think that the authors really believed that C was not a close call. These statistics indicate otherwise.

*In English the end of a sentence almost always carries the most emphasis. HERE, and HERE.

This and another question prompted my survey. In OG 2018, option explanations that consist only of "awkward, wordy, indirect, and confusing" but that have other errors are: 670D. 674A. 696A. 697C and D. 700E. 712C, D, and E. 724A. 727B, C, and E. 734D. 752E. 754A. 757E. 768A and B. 777E. 778A. 787D. 804D.
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B is too difficalt - why bother so much?
D - what is that "of"
E - wrong construction - adjective after the noun

Between A and C I thought C at first, but really why to make sentence more dificult

A is right