The argument doesn’t concern what was the “most” important. We’re trying to establish that certain buyers did not find safety to be important at all.

(B) doesn’t establish what people who rely on ads and promo materials find important. The conclusion concerns something inside the buyers’ mind — do they consider safety important or not? An answer that merely describes what’s in ads and promo materials does not establish what the buyers value or think to be important.

(C) doesn’t establish that the buyers who rely on ads and promo materials don’t find safety important. Although some recent car buyers don’t necessarily tell the truth about purchase factors, that doesn’t guarantee that the people who rely on ads and promos are the ones who aren’t telling the truth, nor does it guarantee that the thing they aren’t telling the truth about is whether safety is important to them.

Even if most consumers know that ads and promos aren’t objective, that doesn’t imply that the people who do rely on ads and promos have such awareness. Maybe they are part of the minority who don’t know that these materials aren’t objective. In any case, even if they were aware the materials aren’t objective, that wouldn’t guarantee that they don’t find safety important.

We know from a premise that people who rely on ads/promos didn’t consult objective sources before buying. So, according to (E), they can’t be among the people who find safety an important factor. If they were among those people, then they would have consulted an objective source.