I went with E here. Here's my breakdown:
The stem tells us that there are 2 groups and 2 products (familiar and unfamiliar). The researchers rushed one group and and did not rush the other. Group that was rushed all went with the familiar product, whereas only 2/3 people in group 2 went for the familiar one. Thus, scientists concluded that the stress caused by rushing was counteracted by the familiarity of the product, for group 1.
How I see this conclusion is that due to a constrained time period to choose, group 1 people went with the familiar product, whereas since group 2 people had time, some of them still chose the unfamiliar one after thinking (since they had the time to think). This has to reflect in the answer choices, as the correct answer would explain this conclusion better.
Option A: Any difference in the frequency or geographic span of the advertising of the product, should have directly affected the familiarity/unfamiliarity of the product. Since it is already given to us that one of the products was familiar and the other one was not, this information will not help us at all. Besides, it does not help us define any relationship between the time taken to make a choice and the familiarity of the product. So we can eliminate this.
Option B: How much a particular group's participant valued the product should not have affected their decision making from a time standpoint. Besides, the only explicit information that we are given about the participants and products, states that the difference in products is on familiarity and the difference in participant groups is the time that was given to them to make a choice. Thus, it would be over-reaching I feel, to be taking this option into account. Hence, Eliminate.
Option C: Again, in this option, what we are explicitly given about the nature of the experiment is that the difference in conditions for both groups lies just in the time given to them to make a choice. It cannot be assumed that the instructions given to both groups were different. Hence, eliminate this.
Option D: The passage exactly tells us that the amount of stress outside the experiment is equal for all participants. The nature of the stress should be immaterial, since the experiment is only evaluating the amount of stress. We can eliminate this.
Option E: This actually helps us establish some relation between the time taken to choose by participants and the nature of the product. Every choice goes through a certain evaluation process for every individual on the basis of a list of parameters. Now if this list was long, every individual would need more time. Since group 1 participants had less time, they could not evaluate the choices on all parameters, they would have to go for a familiar option only. Thus, this explains the conclusion as to why for group 1, the familiarity counteracts the time pressure. Hence, I chose this.