Recent cosmological advances have made it possible to

Official Answer

Since the underlined portion of the sentence contains an idiom, check whether the idiom is used correctly. In this case, the phrase only a decade ago is placed between the words would and have been, even though the idiomatic phrase would have been should not be split. This is an idiom error, so eliminate choice A and look for any obvious repeaters. There are no obvious repeaters, so evaluate the remaining answer choices individually, looking for reasons to eliminate each.

Choice B corrects the idiom error by using the phrase would have been without interruption, and introduces no new errors, so keep choice B. Choice C removes the word it to create the construction made possible but incorrectly follows these words with that one can observe instead of with the observation of, so eliminate choice C for an idiom error. Choice D removes the word it to create the construction made possible, but incorrectly follows these words with the observing of instead of with the observation of, so eliminate choice D for an idiom error. Choice E removes the word it to create the construction made possible, but incorrectly follows these words with the observing of instead of with the observation of, so eliminate choice E for an idiom error.

Choice A: No. The phrase would have been should not be interrupted by other words. Idiom.

Choice B: Correct

Choice C: No. The phrase made possible is incorrectly followed by that one can observe. Idiom.

Choice D: No. The phrase made possible is incorrectly followed by the observing. Idiom.

Choice E: No. The phrase made possible is incorrectly followed by the observing. Idiom.

The correct answer is choice B.