Section A
1. "who"
2. "where"
3. "that"
4. "who"
5. "whose"
6. "that"
7. "which"
8. "whose"
9. "where"
10. "who"
11. "that"
12. "where"
Section B
Relative pronouns are easy to identify, really. You're looking for words like "who", "that", and "whom", as rarely as that particular one is used.
3. You could leave out "that", and the sentence would still be perfectly readable as a complete sentence.
6. Same as number three. You could leave out "that".
11. You could leave out "that" in this one as well, and have no problem.
Section C
1. I don't see any need to add in commas.
2. Again, I see no need to add them.
3. Beijing, which is one of the world's biggest cities, hosted the 2008 Olympic Games.
4. Michael Jackson's Thriller, which was released in 1982, was one of the best selling albums of the 80s.
5. I see no need here.
6. Sally and Joe, who got married last year, are expecting their first baby.