Hi Anali,
Thanks for the question! This problem is a great example of how LSAT questions can be somewhat timeless, as a questions very similar to this one appear on the LSAT regularly.
First off, let's get right into the negation question you ask about (A). It is indeed appropriate to apply to this Assumption question, but the problem you ran into is that you did not negate answer choice (A) correctly. Answer choice (A) in this problem reads as: "Some Minlandians derive no pleasure from watching television in a language other than their native Minlandian." To negate this, you can only manipulate
one of the terms, not both terms as you did. This makes the proper negation: "
No Minlandians derive no pleasure from watching television in a language other than their native Minlandian." Or, in other words, "All Minlandians derive pleasure from watching television in a language other than their native Minlandian." However, the argument is about preference, not pleasure, so this does not undermine the argument.
Moving on to answer choice (D), "
At least some of what the Minlandians read for pleasure is in the Minlandian language," which in it's native form supports the cause. When negated, this answer becomes "None of what the Minlandians read for pleasure is in the Minlandian language," as you correctly noted. If that is the case, then this undermines the cause claimed by the psychologist (which is what we are specifically told to do in the question stem). Here;s the thing about your comment about the premise: negation is a form of manipulation of the answer, and if you negate an answer and it attacks
anything in the argument, choose it! Here, we have an answer that would undermine the claim of the psychologist, and that was the goal.
This also points to a misunderstanding on your part of the advice in the LRB about attacking arguments. What I say in the Bible is that Weaken answers tend to address the conclusion or the step from premise to conclusion. I do
not say they can't ever address a premise, it's just that it's very rare because in Weaken (and Strengthen) questions it tends to be very obvious that that is happening. The test makers don't want to make things easy so they avoid that.
So, let's say for a moment that you had a situation where you negated an answer and it attacked only the premise (or if that happened in a regular Weaken question). It would still be correct because that would in fact undermine the argument. The test makers won't put two answers into an Assumption question that both attack different parts of the argument when negated because both would be assumptions. So, that's why I say if you negate an Assumption answer and it undermines anything, it's correct
Please let me know if that helps, thanks!