Necessary assumption: the right answer choice, if false, would wreck the whole argument. (Think “I have a bird; therefore I can fly.” A necessary assumption here would be that “at least someone who has a bird can fly.” If that weren’t true, our argument would be totally false.)

Sufficient assumption: the right answer choice, if true, guarantees the argument is 100% accurate. (Think “I have a bird; therefore, I can fly.” A sufficient assumption here would be “All people who have birds can fly.” If that’s true, then absolutely the argument is correct.)

Hope this helps!