Necessary Assumption Intro
The goal of Necessary Assumption questions is to find an answer choice that needs to be true for the conclusion given in the stimulus to be valid. It's important to note that unlike Sufficient Assumptions, Necessary Assumptions don't have to prove the conclusion - their truth is just needed for the conclusion to be true.
Check out my article on the Difference between Necessary and Sufficient Assumptions if you haven't already.
Approach
Because Necessary Assumption stimuli rely on an assumption, they are inherently flawed. The first thing you should look for when faced with one of these questions is a flaw in the stimulus.
Some of the common flaws in Necessary Assumption stimuli include failing to eliminate alternative explanations and scope changes.
Failure to Eliminate Alternative Explanations
When you find this flaw in a Necessary Assumption question you should recognize that for the conclusion to be true, we need to assume that the viable alternatives can't be the actual explanation. So look for an answer choice that eliminates at least one of them.
For example, take the following argument:
"Eunice is not at home. Therefore, she's at the basketball court."
This conclusion is not valid because for all we know Eunice could be anywhere. She could be at the store, on a sailboat, at a French language immersion class, etc. The argument just assumes she isn't at any of those other places.
Do the following questions on LawHub or another licensed platform and try to internalize the pattern that I'm describing.
Scope Changes
This flaw is extremely common on Necessary Assumption questions. The stimulus will tend to make a leap from one concept to another, essentially assuming that they are the same. We need to find an answer choice that bridges the gap.
Try the following examples:
Conclusion Only Partly Proven
Sometimes with Necessary Assumption questions only part of a conclusion will be proven. The conclusion may conclude multiple things, or that something applies to multiple groups, but only partially prove that broad claim.
Note that for Necessary Assumption questions we don't necessarily need an answer choice that proves the entire conclusion. The correct answer may help prove the whole conclusion, but we only need the answer to help prove at least one part of it.
Here's an example:
"Every tree that Justin has planted in his yard is of a species he likes. He has an oak tree in his yard, therefore Justin likes, oak, maple, and elm trees."
The conclusion here is 1/3 proven. We proved that Justin likes oak trees, but the evidence doesn't tell us that he likes maple or elm trees. The author just assumes that he likes those species.
So the correct answer could tell us that Justin likes maple and elm trees, but it's also a necessary assumption that he likes elm trees, and it's necessary that he likes maple trees. Even though those assumptions individually don't prove the conclusion they're still needed.