Always we must use a subject after a prepositional phrase: here is barbara and bla bla....so suddenly we have to choose between A and B. Who cares of the rest, you should not see it neither.

Then , the difference is on the last word: major and then what ?? importance. Major if is not followed by importance does not make sense.

If you think about those only rules, one step at time you can answer to this question (difficult indeed) in 20 seconds.

Hope this helps

After long years of indifferent reception by the scientific community, Barbara McClintock’s subtle work on the genetics of corn has now been acknowledged to be of major importance; her analyses show that movable elements control the action of genes and thus cause mutations.

(A) Barbara McClintock’s subtle work on the genetics of corn has now been acknowledged to be of major importance ( Ok)

(B) Barbara McClintock’s subtle work on the genetics of corn has now been acknowledged as being major ( as being major is awkward)

(C) it has now been acknowledged that Barbara McClintock’s subtle work on the genetics of corn is major in importance (misplaced modifier, it has now been acknowledged that... is a mess)

(D) there is now acknowledgement of Barbara McClintock’s subtle work on the genetics of corn as of major importance ( misplaced modifier, there is now acknowledgement of... is a mess. Furthermore, major in importance is awkward)

(E) the importance of Barbara McClintock’s subtle work on the genetics of corn has now been acknowledged to be major (misplaced modifier, the opening phrase illogically modifiers the importance of Barbara McClintock's subtle work rather than Barbara McClintock's subtle work. moreover, to be major doesn't make sense, it should be to be major of something )

GMATPrep Code : VSC002602