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.
Difference between Sufficient and Necessary Assumptions
Consider the example below:
"Jones is at the t-shirt store and the t-shirt he wants costs $20. Therefore, Jones can afford the t-shirt he wants."
See the assumption? We need to assume that Jones has $20! Otherwise how would Jones afford the t-shirt?
But this example involving Jones actually goes even deeper than that. Consider: wouldn't we also need to assume that Jones has at least $10? Or at least $5? Or just that Jones has some amount of money? These are all necessary assumptions. Even though the ones involving less than $20 don't prove the conclusion, they are still needed for the conclusion to be true.
On the other hand, what if we assume that Jones has $30? This proves the conclusion, so it's a sufficient assumption. But it's certainly not necessary to have $30, because the t-shirt only costs $20.
In the example there is one assumption (that Jones has $20) that is both sufficient and necessary. But there are also nearly endless assumptions that are either sufficient or necessary. We need to assume that Jones is not flat broke, but that doesn't prove he can buy the shirt. It would be sufficient to assume that Jones has a $100 bill, but that's not necessary considering the t-shirt is only $20.
I find that students often find it hard to understand the difference between sufficient and necessary conditions. Hopefully this example helps make a distinction between the two types of assumptions and what you are looking for when evaluating these question types.