Hi Scarlett,
This is a concept that we call The Justify Formula. It is discussed in lesson 4 of The PowerScore LSAT Course and in chapter 10 of "The Logical Reasoning Bible."
You can also find information about it here:
https://blog.powerscore.com/lsat/unders ... questions/
And in episode 12 of the podcast, which can be found here:
https://blog.powerscore.com/lsat/lsat-p ... questions/
The basic idea is that the correct answer to a Justify question, when added to the premises already given in the argument, will 100% get you to/prove the conclusion.
Since arguments in Justify questions often involve conditional reasoning, the correct answer often involves a conditional statement with a term from a premise in the sufficient condition and a term from the conclusion in the necessary condition.
Here's a very simple example to illustrate how it works. Imagine we have the following argument.
Premise: John has a Snickers.
Conclusion: Therefore, John has a candy bar.
What answer would prove this conclusion 100%?
The answer is:
If someone has a Snickers, then that person has a candy bar.
If you add this statement into the argument, it 100% gets you from the premise to the conclusion. In other words, it Justifies the argument. Of course, the correct answer could be stated in the form of the contrapositive of what we are looking for (as it is in question 22), so be on the lookout for this as well.
If the answer had been stated backwards, what we call a Mistaken Reversal, it would not Justify the conclusion because it would not get us from the premise to the conclusion.
The following answer would be incorrect:
If someone has a candy bar, then that person has a Snickers.
Of course, arguments can get much more complex than the example given, but the idea is the same. Properly diagramming the conditional statements (when there is conditional reasoning) will help spot the missing "link" in the logical chain.