No outside knowledge of the world is really required on the LSAT. All the information you need, for the most part, is already on the page. You do need to think critically about what you read, of course, but you don't need to have prior knowledge of the topics that are being discussed. Question 19 in Section 2 of the June 2007 LSAT is a good illustration of this. You definitely don't need to know anything about the nation of Banestria, and if you do know something about Banestria it can surely only hurt you. Because "Banestria" doesn't even exist. The LSAT is a game. It frequently features made-up people, in made-up countries, doing made-up stuff. As I've written, the central goal in the game is fill-in-the blank: "The argument is bullshit because ______________." It's okay to have fun playing this game! The more fun you have, the better you'll probably do.
Here, the historian's conclusion is about Causation. (Here's an easier question about Causation--you might want to start here, if you haven't already.) The historian says the Land Party's success in 1935 was due to, i.e. caused by, "the combination of the Land Party's specifically addressing the concerns of [agricultural and small business] groups and the depth of the economic problems people in these groups were facing." Now let's look critically at the evidence for that assertion:
1) The Land Party specifically targeted agricultural and small business groups in 1935, and this was that year was the only year the Land Party won a national victory. Okay, but how do we know that the Land Party didn't target those groups in other years as well? How do we know that the Land Party didn't always target these groups? Moreover, how do we know that every party doesn't always target these groups?
On a different track, do we even know that the Land Party ran national campaigns in any other year? Maybe 1935 was the only year they ran, and the only year they won! The facts seem to imply that there's a correlation between the economic events and political strategy of 1935 and the Land Party's sole victory, but implying things on the LSAT is not good enough. The argument has huge holes in it.
2) The Land Party got most of its support that year in rural and semirural areas. Yeah, but isn't that probably true of all parties? It says that's where the bulk of the population lived at that time! Furthermore, it's probably reasonable to assume that the agricultural groups lived in rural and semirural areas, but how do we know that small business groups weren't mostly in the cities? More holes.
3) The economic woes of the years surrounding that election hit agricultural and small business interests the hardest. Okay, but how do we know that they voted for the Land Party because of these economic woes? Couldn't they have just voted for the Land Party because the Land Party had the sexiest candidate? Or because all the other candidates from other parties were more horrible than usual? Maybe everyone was going to vote for the Land Party anyway, regardless of the specific targeting and the economic woes? Even more holes.
As you can see, this argument is bullshit for a lot of reasons. It's barely even coherent, to be honest. Any time you see causation on the LSAT, you need to start asking questions.
The question says "Each of the following, if true, strengthens the historian's argument EXCEPT." This means four of the answers (the incorrect ones) will each strengthen the argument. The correct answer might weaken the argument, but it doesn't have to. The correct answer could also be something completely irrelevant here. If an answer choice says "Jim Nantz is a God-awful sports announcer," I will definitely pick it. It's definitely true in real life, but it can't possibly strengthen the Historian's argument. Let's see what we've got here:
A) Hmm. If this said "economically distressed rural and semirural groups" then it would strengthen the argument by addressing one of my questions above. But since this answer is about urban groups, I'm not sure it's relevant to the historian's argument. What do urban groups have to do with anything? They're not mentioned anywhere in the argument. This could be the correct answer to an EXCEPT question.
B) This answer strengthens the connection between "the Land Party targeted these groups" and "this caused the Land Party to win." This is a strengthener, so it's not the answer to this EXCEPT question.
C) This answer strengthens the connection between "the economy for these groups was really bad" and "this caused the Land Party to win." This is a strengthener, so it's not the answer to this EXCEPT question.
D) This answer strengthens the connection between "the Land Party targeted these groups" and "this caused the Land Party to win." This is a strengthener, so it's not the answer to this EXCEPT question.
E) This answer strengthens the connection between "the economy for these groups was really bad" and "this caused the Land Party to win." This is a strengthener, so it's not the answer to this EXCEPT question. Our answer is A, since it was about the wrong group of people and is therefore irrelevant to the argument.