These questions will show up after the details and main idea questions. The wording can vary somewhat, but it will ask which of the answer choices, if true, would either strengthen or undermine a claim made in the text. Look out for the words "if true", and words like strengthen, support, weaken, undermine, etc.
There is always some claim, hypothesis, or conclusion that the question is asking you to evaluate. The first thing you'll want to do is identify exactly what that claim is. Sometimes they are nice and underline the sentence that contains the claim. Typically, the claim is in the last sentence or two of the passage.
Remember that you aren't always looking to support the claim - sometimes you'll be looking to do the opposite. Make sure you carefully read the question to see which direction you're looking for.
Once you've identified the claim, you'll want to read the passage to gain some extra context around that claim. As you go, try to summarize each sentence in your own words in your head (or with the annotation tool), and ask yourself how it might relate to the claim.
There will usually be some information in the passage that is used as rationale to arrive at the claim. Carefully evaluate what pieces of information their claim relies on. The correct answer is probably going to either support or undermine the foundations on which the claim is built, rather than directly reinforcing or contradicting the claim itself.
Before looking at the answer choices, see if you can think of scenarios that would support or undermine the claim (depending on which the question is asking you to do). It's ok if you can't, but this thinking can help ground your mind as to what you might be looking for.
Now it's time to go through each answer choice and use process of elimination to find the right answer. For each one, ask yourself it it would accomplish the goal of either strengthening or undermining the claim. Remember that the correct answer will often relate to the evidence used to arrive at the claim, so make sure not to dismiss any answer choices prematurely. If they reference something from the text, make sure you have an idea of how that information in the text relates to the claim.
On these questions, you are alwasy operating under the assumption that the each answer choice is true. So unlike most Reading & Writing questions, we're not concerned with whether the answer choices are supported by the text. We just care whether, if they do happen to be true, they would accomplish the goal of either strengthening or undermining the claim.
This goes back to the first step - make sure you carefully read the question to see which direction you're looking for. A very common mistake is to choose an answer that supports the claim, when the question is actually asking you to undermine it. So make sure you are clear on which direction you're looking for, and that you pick an answer choice that achieves that goal.