OFFICIAL EXPLANATION
The stimulus is prefaced by the word “advertisement.” One quirk of the GMAT is that every stimulus in GMAT history that has been preceded by this word has contained faulty or deceptive logic. Thus, whenever you see this word prefacing a stimulus, be on the lookout for misleading or flawed reasoning. The argument is constructed as follows:
Premise: At most jewelry stores, the person assessing the diamond is the person selling it.
Premise/Sub-conclusion: So you can see why an assessor might say that a diamond is of higher quality than it really is.
Premise: All diamonds sold at Gem World are certified in writing,
Conclusion: You’re assured of a fair price when purchasing a diamond from Gem World.
The first sentence contains a premise and conclusion that relies on the assumption that financial motivation might cause a person to lie about the quality of the item. According to the advertisement, at Gem World there is no such worry because the diamonds are certified in writing. Think for a moment—does that reasoning sound bulletproof? If you were standing there in the store and you were told that Gem World has written certification, wouldn’t you ask who does the certification? This is the essence of personalizing the argument—place yourself inside the situation and think how you would react. As soon as you do that in this question, the weakness in the argument becomes apparent. Then, since this is a Strengthen question, you can look for an answer choice that eliminates this weakness. Answer choice (E) addresses the hole in the argument by indicating that the individuals who provide the written certification are not the same people who are selling the diamonds at Gem World.
There are other errors in the stimulus, such as assuming that a written certification equals a fair price. The certification may have no impact on the actual price of the diamond, or perhaps it could even be used to raise the price unjustly. These problems are ignored by the answer choices, and the test makers have that right.
Answer choice (A): The conclusion addresses the fair price of diamonds at Gem World, not other stores. Hence, the fact that other stores have written certification does not help the Gem World advertisement.
Answer choice (B): This is an answer many people keep as a Contender. The answer is incorrect because it fails to address the point raised in the first sentence, namely that the person assessing the diamond has a personal stake in the outcome. This “accountability” issue is the central point of the argument, and without knowing the source of the certifications, this answer does not strengthen the argument.
Answer choice (C): The argument asserts that a fair price is assured when purchasing a diamond at Gem World. No claim to comparative quality is made in the advertisement, and thus this answer does not strengthen the argument.
Answer choice (D): If anything, this answer may hurt the argument since it indicates that a fair price may not be obtainable at Gem World due to price volatility. If prices change daily, then Gem World may be selling diamonds at a price that does not reflect current market value. However, the answer choice specifically mentions “the most expensive diamonds” and there is no guarantee that Gem World carries diamonds in this price range. So, at best, the answer choice has no effect on the argument and is therefore incorrect.
Answer choice (E): This is the correct answer. As mentioned above, this answer addresses the separation of the certification writer from the seller and thereby strengthens the reasoning.