The preeminent astronomer Carl Sagan posited that the expansion of

Because whole sentence is underlined we have to carefully examine all answer choices:

(A) The preeminent astronomer, Carl Sagan, posited that the expansion of the universe, with galaxies rushing ever outward away from each other, was caused by the Big Bang about 15 billion years ago.

placement of modifier “with galaxies” is incorrect - this is not the intended meaning of the sentence
For more information about with + [noun] + [participle] please see Mister Mike's explanation

Meaning of the sentence: Carl (astronomer) affirmed that the expansion of the universe was caused by the Big Bang about 1000000000000 years ago or so. This expansion then caused some galaxies run away from each other



B) The preeminent astronomer, Carl Sagan, posited that the expansion of the universe, with galaxies rushing ever outward away from each other, is caused by the Big Bang about 15 billion years ago.

Same issue with (A) - modifier placement error. Additionally we have verb tense error, explosion was 1000000000000 years ago, why is?????



C) Carl Sagan, the preeminent astronomer, posited that the expansion of the universe was caused by the Big Bang about 15 billion years ago and the galaxies are rushing ever outward away from each other.

"and" //ism marker, but we don't have anything in the sentence to be // to the galaxies



D) The preeminent astronomer, Carl Sagan, posited that the Big Bang caused the expansion of the universe about 15 billion years ago, sending the galaxies rushing ever outward away from each other.

Beautiful structure, clean and tidy, all verbs and subjects are in place and agree in number, verb tenses are also fine - past simple. Modifier ",+sending" smoothly describes and gives the result of the expansion process, and modifies preceeding clause. "sending" - represents current action also, becouse expansion happened 1000000000 years ago, but still going on. I'm happy

For more information about V-ing modifier please see e-gmat explanations



E) The preeminent astronomer, Carl Sagan, posited that the Big Bang, causing the expansion of the universe about 15 billion years ago, sending the galaxies rushing ever outward away from each other.

As Sir Charles said, such mess here that I didn't know how to explain it
There is no verb for "Big Band", even if we consider "causing" as a modifier of "Big Bang, but what about ", + sending", what is modifies here? it's not // to "causing" - no //ism marker. Anyway (E) is out.



(D) is the answer