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