Because of a rare type of fungus that killed off many cacao trees in B

Remember that when doing weakening questions, 4 of the answer choices WILL NOT WEAKEN the conclusion. 1 answer choice WILL WEAKEN. The reason the question asks which of the following will MOST WEAKEN the argument is because they don't want people to argue that TWO solutions may possibly weaken the conclusion. Hence, only one of the choices WILL WEAKEN the statement. If someone seeks to argue another answer choice, the test makers have covered their bases with the "MOST" word, and say that their answer choice is the one that MOST weakens.

With that in mind, when approaching a weaken question, ALWAYS identify the conclusion, which is "the retail price of chocolate is certain to increase within six months."

Now, out of the following choices, which one will attack this statement? In other words, assuming that the following are true, which will NOT make the retail price of chocolate certain to increase within 6 months?

A) Consumers will purchase other sweets if the price of chocolate increases.
Doesn't really attack the conclusion. The retail price will still increase before consumers purchase other sweets.

B) Researchers have discovered an effective method to kill the fungus.
Specifically talking about this year's crops which have already been harvested, so the fungus method would not affect this year's crops.

C) Dark and bittersweet varieties of chocolate will be affected more seriously than milk varieties.
Not relevant as we are not concerned with types of chocolate.

D) The price of chocolate has decreased steadily for three years.
Past years do not affect this year's crops, and we are told that this year there's a harvest shortage.

E) Most chocolate in stores is manufactured from cocoa that was purchased two years earlier.
If MOST chocolate is manufactured from previous cocoa, then this year's crops MAY NOT NECESSARILY increase the price of chocolate. Yes, it is still POSSIBLE that SOME chocolate retail prices will increase, however given that MOST of it will not, this weakens the author's claim that the price of chocolate is CERTAIN to increase in the next 6 months.

I think the key word is CERTAIN, because given that most cocoa beans are taken from a previous crop, it is NOT CERTAIN that the price of chocolate will increase in the next 6 months. This is the only answer choice that casts doubt on the certainty of the price increase is E). Again, we are searching for something that casts doubt on the certainty of the price increase, and the only answer choice that does so is E).