Relative Clauses

Which of the following is correct?
1. At the conference, I met other Illinois presenters, several who were from NIU.
2. At the conference, I met other Illinois presenters, several whom were from NIU.
Which of the following is correct?
1. My friend, who is very optimistic, thinks that she will graduate this fall.
2. My friend who is very optimistic thinks that she will graduate this fall.
3. My friend that is very optimistic thinks that she will graduate this fall.
Which of the following is correct?
1. The man, whom everyone suspected, was later arrested for burglary.
2. The man which everyone suspected was later arrested for burglary.
3. The man, who everyone suspected, was later arrested for burglary.
Which of the following is correct?
1. He introduced me to his friends, one of which was in my class.
2. He introduced me to his friends, one of whom was in my class.
Choose the correct sentence:
1. Whichever answer you choose, you need to be sure of it.
2. Which ever answer you choose, you need to be sure of it.