D. Write 'm not, aren't or isn't.

Solution:

1. She isn't ugly. She's beautiful.
2. It is cold. It's hot.
3. Your shoes are dirty. They're clean.
4. The boy isn't bad. He's good.
5. We aren't young. We're old.
6. I am sad. I'm happy.

Answer: isn't, is, are, isn't, aren't, am