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).