Explanation:
Depending on what discussion your having, if your helping/tutoring somone, 3. If you are giving a reason or just talking to somone, 2. And if someone is talking during your discussion or somone else's, 4. 1 can be used when trying to start up a conversation with two other people.
Answer:
"What puzzles me is when the author..."
Explanation:
This is the answer that I am thinking. It makes the most since because it could really go along with a discussion.
Hope this helps!