Bunuel wrote:
The Grand Canyon, which was previously thought to have been carved out by the Colorado river, was eroded away by a lava flow, according to a newer theory, took on its current form at an earlier date 6 to 7 million years ago.
A. which was previously thought to have been carved out by the Colorado river, was eroded away by a lava flow, according to a newer theory,
B. previously thought to have been carved out by the Colorado river eroded away by a lava flow, according to a newer theory, and
C. previously thought to have been carved out by the Colorado river, was eroded away by a lava flow, according to a newer theory, and
D. previously thought to have been carved out by the Colorado river, according to a newer theory, it eroded away by a lava flow and
E. previously thought to have been carved out by the Colorado river, according to a newer theory, which was eroded away by a lava flow and
Official Explanation
Creating a filter: in this question, the original sentence is, in fact, not grammatically a sentence. The last phrase, starting with "took on its current form," is a predicate with no grammatical subject, since the phrase "the Grand Canyon" has already gotten a predicate earlier in the sentence in the phrase, "was eroded away by a lava flow." Therefore, answer choice (A) is out. We can look for similar issues in the predicate of the remaining options.
Applying the filter: choice (B), somewhat like (A), diverges from the intended meaning of the sentence by indicating that the Colorado River was eroded away by a lava flow. Choice (B) is out. Choice (C) is improved; the "and" makes the end of the sentence part of the same predicate as "was eroded" and we have a clear sentence. So we can come back to (C). Choice (D) is a run-on sentence / non-sentence due to the inclusion of the pronoun "it" with no conjunction. Moreover, it provides no verb for the first subject, "the Grand Canyon." Choice (D) is out. In (E), the original subject of the sentence, "the Grand Canyon," never gets a verb.
The correct answer is (C).