What a sneaky little question, playing on my assumptions so well
I got this question wrong, but let me see if I can work it out.
The intro in this passage really sets you up on this one, trying to trick you into making the same assumption the author makes. It talks about how increases in hearing loss among teens is due to their listening to loud music.....So I automatically assumed....oh yeah, teenagers, irresponsible, don't care about the long term effects on their hearing, only care about short term enjoyment of music....
But the beginning was just irrelevant context. The meat of the passage was in the last sentence.
Conclusion: is the adoption of the recommendation by manufacturers to make and sell stereo headphones which automatically turn off when a dangerous volume is reached will not greatly reduce the occurrence of hearing loss in teens.
Premise: Why? because almost all stereo headphones used by teens are bought by teens.
There are so many assumptions going on between the premise and conclusion, such as: if these new type of stereotypes were cheaper, teenagers would not still buy them. If teenagers became aware of the risks involved in listening to dangerously loud music they would not consider more safer stereo headphones (notice how the introduction tries to set you up for ignoring this assumption, trying to trick you into assuming teens are too irresponsible to care). Another assumption is that parents don't greatly influence teenagers' buying decisions and would not successfully pressure them into buying these new kind of headphones....and the list goes on.
D eliminates one of these assumptions as a source of attack and thereby strengthens it, saying that even if teenagers knew the risks of listening to loud music, they would still continue to listen to loud music. This offers the argument a supporting premise as to why these new type of headphones would not greatly reduce hearing loss among teens-- because even if teenagers knew about the dangers of listening to loud music they would still listen to loud music and therefore may not be as likely to buy the safer headphones as they would if this assumption were untrue.
I chose C, because I was thinking---well ok the parents are all for these new types of safe head phones----which means that they would possibly buy them----but that doesn't mean they don't listen to loud music---so why would teenagers who listen to loud music-----not potentially buy the new head phones?(complicated I know, I have no idea why I reasoned it that way)
I was getting to the heart of the assumption, but I missed it by a lot. First, the author does not explicitly state that the parents would be willing to buy these new type of head phones, that was an assumption I was making based on the fact that they were pushing the manufacturers to make safer headphones. A reasonable assumption I know, but out of scope in the context of this LSAT passage and nevertheless an assumption. Second of all, D is the right answer because it gets more to the heart of matter - it explicitly connects the premise to the conclusion by referring to the teens themselves. C does not do this. The strength of the premise for the support depends on the will of teenagers to buy these new type of headphones, not about parents. And just because parents are willing to do something does not mean that teenagers are willing to do the same thing.