Many winemakers use cork stoppers; but cork stoppers can leak, crumble

So this is a pretty lengthy argument, but it is one argument that progressively advances toward the main conclusion. That eliminates answer choices A and B immediately, because there is nothing contradictory to the argument that is presented. The argument can be summarized as follows:

P) Cork stoppers have problems that result in wasted inventory
P) Bottlemaster plastic stoppers do not have these problems
P) Bottlemaster plastic stoppers are slightly more expensive than traditional cork stoppers
P) Cork prices are going to rise dramatically
Intermediate Conclusion) Winemakers who use cork but wish to keep production costs low will have to reconsider plastic stoppers
P) Public's negative association with plastic wine stoppers is declining
Ultimate Conclusion) Bottlemaster plastic stoppers will gain an increased market share

Veritas teaches a great technique of the "why test." Any conclusion must be supported with reasons in the argument, and the ultimate conclusion here is supported by the fact that the public is no longer so negative about plastic stoppers and winemakers who currently use cork stoppers but who wish to keep production costs low are going to need to reconsider using plastic stoppers. Veritas also teaches something that I refer to as the "reverse why test." This states that any ultimate conclusion of an argument cannot serve as the "why" to something else - it is in no way a premise. I like to think of it as the "end of the line." Here the intermediate conclusion (as we just saw) is also a conclusion, because it is supported by the fact that the plastic stoppers are a good substitution for the cork stoppers, and cork stoppers are going to rise drastically. It then itself, however, serves as a "why" to the main conclusion, as we just saw, meaning that it cannot itself be the main conclusion - it does not pass the "reverse why test."

That makes answer choice E the correct answer. Both C and D incorrectly cite the intermediate conclusion as the main conclusion.

I hope this helps!