(A) makes a probabilistic claim about the future, whereas (B) makes a definite claim about the future. Practically, that should be the difference-maker as you choose between the two. Here’s the logic we can use to prove (A), though:

Recently-published books that are insignificant will probably never be deacidified, which means they’ll probably always have acid in them, which means they’ll probably keep deteriorating forever, which means they’ll probably deteriorate completely.

This requires some pretty specific logic that isn’t immediately obvious. Most importantly, the scope of the stimulus’ last sentence is incredibly broad – it applies to all future deacidification methods. Here’s its translation, which is also listed as claim 5 in the analysis section:

Probably: if a book is insignificant, no deacidifying methods will ever be applied to it.

This reading is essential. It tells us that the kind of books (A) discusses – recently published books that are insignificant – will probably continue having acid in them forever.

From there, we can infer (from claim 2 in the analysis) that these books will probably keep deteriorating forever. Last, we need to add in an inference ourselves:

If something deteriorates forever, it will eventually deteriorate completely.

This isn’t an assumption, though. We’re not adding anything new; we’re just recognizing a fact inherent in the concept of deterioration. (Technically speaking, this is called a “corollary.”)

(B) makes a definite claim about the future. Practically speaking, the “definite” part should help you choose (A) over (B) in a timed setting, because (A)’s probabilistic claim is comparatively easy to prove.

The “future” part is key to eliminating (B). (B) doesn’t say these books are gradually destroying themselves, it says they will gradually destroy themselves.

These books are, for sure, currently destroying themselves with their own acid. In the future, though, there’s room for a lot of them to have their acid removed, and to thereby stop destroying themselves.

Perhaps lots (or all) of them have historical significance. Or perhaps this “only significant books get deacidified” prediction won’t even come true. The premise says it will probably happen, but perhaps it won’t happen and all these books will get deacidified whether they’re significant or not.

This is just the converse of the claim the stimulus’ second sentence – it’s backward. Reversing conditional statements isn’t a valid move.

We don’t know what kind of paper books published before 150 years ago used. Perhaps none of them used acidic paper at all, meaning none of them will require deacidification.

We have active reasons to believe this is false. It may be that some of these books are stored in a cool environment, slowing their deterioration, whereas others aren’t.