Here are the correct answers:

1. **C. but**
2. **A. but**
3. **C. or**
4. **A. however**
5. **D. otherwise**

So, the complete sentences would read:

1. Teenagers need encouragement from their parents, **but** not all parents are willing to encourage their children.
2. Many girls worry about their appearance, **but** they often look at themselves in a mirror.
3. Teenagers should learn to mix with their classmates, **or** they can try to get on with their siblings.
4. Social media help teens connect with others; **however**, they also cause teens to feel lonely.
5. They spend a lot of time surfing the net; **otherwise** they have little time to read books.