I'm going to start by quoting Adam's explanation about answer choice (B) from earlier than this thread:
Adam Tyson wrote:Try that negation of B again, LSATer, and I think you'll find that it actually strengthens the conclusion of the argument, proving that the answer cannot be an assumption of the argument.
Our author is trying to prove that the food you eat will cause your food preferences to change. If I feed you salty food long enough, you will eventually come to prefer it over sweet food, even though you previously preferred sweet food. I think of it as sort of a "Stockholm Syndrome" of food preferences.
If food preferences usually change between one and two years of age, then this transformation over the course of that year for the children studied might have nothing to do with having been fed salty food over the course of that year. It could have just happened on its own due to those natural changes. When I negate B, I say that preferences don't usually change over that time, but I know from the experiment in the stimulus that these kids did have a change. They were apparently out of the ordinary, if the negation of B is true. This eliminates random chance or ordinary natural changes as possible alternate causes for the changing preference exhibited by these kids. Eliminating alternate causes is one of our favorite ways to strengthen a causal argument!
We don't want it to be true that tastes usually change - we want it to be odd that they changed, giving us more reason to believe that the change was due to the prominence of salty food. Otherwise, the diet over that year might have had no impact at all and been a complete waste of time and effort.
Take another look and see if you see it now. Keep at it, you'll get there!
To add to Adam's response: remember that we should accept the premises as true. We know that if you feed a one-year-old a salty food instead of a sweet food then over the period of about a year their preferences will change to salty over sweet. That's the premise, we accept it as true. The conclusion is the last sentence: that a young child's taste preferences can be affected by the food that they are exposed to. The conclusion provides an explanation as to why a child's tastes can change between one-years-old and two-years-old. So when you're thinking about what would attack the argument, it needs to attack the explanation of why young children's tastes change, not whether they change at all. We already know their tastes can change. The argument is about whether they change based on exposure to certain foods.
So, as Adam says, when you negate answer choice (B) to say that a child's taste preferences do not usually change between the ages of one and two, this does not attack the argument. We already know that they can change. Saying that they usually
do not change just strengthens the conclusion that it's the exposure to food that is changing their preferences rather than the change being just a natural side-effect of growing older.
Hope this helps!