Climatic shifts are so gradual as to be indistinguishable at first fro

Climatic shifts are so gradual as to be indistinguishable at first from ordinary fluctuations in the weather.

(A) so gradual as to be indistinguishable
ANSWER: This uses the idiom correctly. "So gradual as to be indistinguishable" is correct. We can say "His work is so famous as to be a cliché."

(B) so gradual they can be indistinguishable
PROBLEM: We need something after "gradual" to segue here. For example "that" or "as".

(C) so gradual that they are unable to be distinguished
PROBLEM: No reason to use the passive voice on "distinguished", because we don't know who is doing the distingushing. Also, "they are unable" makes it sound like they're TRYING to be distinguished, and can't do it.

(D) gradual enough not to be distinguishable
PROBLEM: There's no reason to use "enough" here. It marks a subtle shift in meaning. We want to emphasize that shikts are so gradual that we can't tell the difference between them and ordinary fluctuations. This version, with "enough", makes it sound very direct, as if there's some level of "gradual-ness" that makes shifts indistinguishable from ordinary fluctuations. Finally, putting the not in there instead of the word "indistinguishable" confuses things. It would be weird to say "I am tired enough not to be able to stay awake on my feet" as opposied to "I am tired enough to fall asleep on my feet".

(E) gradual enough so that one cannot distinguish them
PROBLEM: Even worse than above. We still have enough, but we've also added an unnecessary pronoun ("one")