Strengthen with Necessary Premise is one of the more involved concepts on the test, but let's simplify it.
A premise is considered necessary, if and only if the falsity of the premise guarantees the falsity of the conclusion. This is a condensed version of the definition provided in the lesson. The idea behind this is that when you negate a necessary premise, it will weaken the argument because that premise was, in fact, necessary.
An idea that might help simplify this concept is that providing a necessary assumption in a strengthen question will actually strengthen an argument, no matter how slightly. This is because it makes the reasoning more concrete and fills in gaps.
It is not best practice to negate all answer choices for Strengthen questions, rather if you think that one of the answers is a necessary premise, then try negating it and see the impact this has on the argument.