A great question, and one that is not easily answered, so if anyone has a better answer than mine (or one that is more clear), please don't hesitate to chime in.

Answer All is most likely the best option. Why? Because you want a individual to answer all your questions.

Required is a tricking setting to understand. It seems like it should be the same as Answer All, but it is different in one respect (as far as I can tell). I believe under the Required setting, individuals can skip the questions if they wish. They will fail or forfeit them, but they can skip them. You may never see this, as most people don't like failing because they peeked at the next question, but it can come up.