In German, the use of "du" (the informal "you") is typically reserved for friends, family, and people with whom you are on familiar terms. Based on your list, the person you might only refer to using "du" would likely be:
- Die Oma (grandmother)
This is because a grandparent is usually someone with whom you have a close, informal relationship. The other individuals listed, such as a policeman, doctor, or professor, typically would be referred to in a more formal manner using "Sie."