These questions ask you to help the main conclusion:
Because it’s impossible to strengthen a valid argument, there must be at least one flaw in a Strengthen question’s argument. Pinpoint at least one flaw when you actively engage with the passage the first time you read it:
There might be multiple problems with the argument. Identify as many as you can. After spotting one or two serious flaws, jump into the answer choices.
The correct answer won’t necessarily prove the main conclusion, but it will make that conclusion stronger. As you read each answer, ask yourself: Does this answer help the main conclusion more than the other four answers?
While all five answers can be true, only the correct answer will give you new evidence which might have been assumed but never explicitly stated by the author. The new evidence should fix, at least partly, one of the problems you identified.
If two answers both help the conclusion, the more strongly-worded answer is usually the correct one in a Strengthen question. For instance, it might say “all” rather than “some.” As always, though, the content of the answer matters more than word strength.
If a question uses the word “EXCEPT,” look for the opposite: The correct answer will either weaken the conclusion or do nothing, so rule out answers that strengthen the conclusion at all.