The key to necessary assumption questions is understanding that the answer choice, if it is not true, would destroy the argument. Go through every answer choice and ask yourself "does that have to be true for this argument to work?"

If my argument was "Every person likes pie. So, Johnny must like pie." the necessary assumption I am making is "Johnny is a person." If Johnny is not a person, my argument simply does not work. After all, if Johnny is not a person, we cannot say he likes pie.

This is a common tactic for necessary assumption questions. Negate the answer choice. Does the resulting negated statement completely wreck the argument? In this example, the negated assumption would be "Johnny is not a person." Well, that kind of destroys the argument. Look for the answer choice that, when negated, would destroy the argument or contradict it.

Another thing you can do is to ALWAYS identify the conclusion and the premises supporting it. "Every person likes pie" is my premise and "Johnny must like pie" is my conclusion. Before you even read answer choices for necessary assumption questions, try to find the gap for yourself. This will come with more practice, but there will always be a gap in these arguments. What does the argument leap over to get to its conclusion? Sometimes, you'll anticipate the gap and immediately find the corresponding answer choice.

Hope this helps. I recommend drilling necessary assumptions from the easiest question types to apply these tips. Then, work your way up. The higher the difficulty, the more subtle the assumptions will become.

This is why I always read every answer choice. I initially chose B on #8, but when I read C it seemed plausible as well. Re-reading B revealed the trap. Glad I caught it!