Recent cosmological advances have made it possible to

'it' is required. 'make possible' is awkward. A is wordy and awkward. So, B.

(A) it possible to observe stellar objects that would only a decade ago have been unable to be viewed
(B) it possible to observe stellar objects that would have been invisible only a decade ago changing meaning
(C) possible that one can observe stellar objects invisible only a decade ago "made it possible that" is correct construction
(D) possible the observing of stellar objects that were not capable of being viewed only a decade ago too long and wordy,"visible" can replace the phrase "capable of being viewed"
(E) possible the observing of stellar objects that only a decade ago would not have been able to be viewed

B is at least grammatically correct,but is heavily changing the meaning of question.
although i chose E because i could not immediately pinpoint the "observing of" error and rest of the options looked wrong from first look.

B. I was between A and B but A seemed awkward. I found this one tricky

B is clearer. Can someone explain the tense use in the second clause in B. i. e. use of "would have been".

Recent cosmological advances have made it possible to observe stellar objects that would only a decade ago have been unable to be viewed.

(A) it possible to observe stellar objects that would only a decade ago have been unable to be viewed
(B) it possible to observe stellar objects that would have been invisible only a decade ago
(C) possible that one can observe stellar objects invisible only a decade ago
(D) possible the observing of stellar objects that were not capable of being viewed only a decade ago
(E) possible the observing of stellar objects that only a decade ago would not have been able to be viewed

In A, what does "only a decade ago" modify? It should modify the viewing right? But it is placed so far from vieiwed. Second, if we exclude the mistake with only a decade ago, we get ...that would ...have been unable to be viewed. It sounds correct, but its wordy and unnecessary and passive?. Unable to be viewed

1) made it possible vs. made possible by --> I think both are correct. But "made possible" that is wrong. So, rule out C.
2) D and E use the construction --> made possible ...x....that
3) D has introduces capable which distorts the meaning. Capable of being viewed? This is also passive voice. Unnecessary
4)

IMO B, as it sounds right, is 'made it possible' not the prefered option than 'made possible' does this fall under any idiom

Will go with B. "It" appears to be correct.
"made possible" sounds bit weird.

Recent cosmological advances have made it possible to observe stellar objects that would only a decade ago have been unable to be viewed.

(A) it possible to observe stellar objects that would only a decade ago have been unable to be viewed
Wrong. Passive voice is not necessary.

(B) it possible to observe stellar objects that would have been invisible only a decade ago
Correct. "that" modifies objects.

(C) possible that one can observe stellar objects invisible only a decade ago
Wrong. change meaning.

(D) possible the observing of stellar objects that were not capable of being viewed only a decade ago
Wrong. bad grammar structure.

(E) possible the observing of stellar objects that only a decade ago would not have been able to be viewed
Wrong. Passive voice is not necessary.

the correct idiom is "make it possible" -> C,D,E is out.

A is so wordy compared to B -> choose B

(A) it possible to observe stellar objects that would only a decade ago have been unable to be viewed
have been unable to be viewed is wordy. ( invisible is the concise word)

(B) it possible to observe stellar objects that would have been invisible only a decade ago
Correct.

(C) possible that one can observe stellar objects invisible only a decade ago
Wrong. It is too concise to change the meaning.

(D) possible the observing of stellar objects that were not capable of being viewed only a decade ago
Wrong. VAN rule of manhattan gmat.
Verb > Adjective > Noun.
Here observing is used as a gerund, and is less preferable to the simple verb form used( to observe)

(E) possible the observing of stellar objects that only a decade ago would not have been able to be viewed
Wrong. Same as Option D

Between A and B. I choose B because hypothetical past is suited here. B is right.

Recent cosmological advances have made it possible to observe stellar objects that would only a decade ago have been unable to be viewed.
Please don’t take this problem to heart. Not even close to GMAC standards.
Just try to learn something and try to make it correct. And PASS.

(A) it possible to observe stellar objects that would only a decade ago have been unable to be viewed
Redundant passive construction.
Incorrect placement of “only a decade ago”.

(B) it possible to observe stellar objects that would have been invisible only a decade ago
Best of All.

(C) possible that one can observe stellar objects invisible only a decade ago
“stellar objects invisible only a decade ago” – Were they actually invisible or what? - Incorrect
Change in meaning.

(D) possible the observing of stellar objects that were not capable of being viewed only a decade ago
Redundant passive construction
“objects that were not capable of being viewed”

(E) possible the observing of stellar objects that only a decade ago would not have been able to be viewed
Redundant passive construction