Which style (informal, formal)

Which style (informal. Formal) would you use in an email to:
1 the director of studies at a science university?
2 a classmate from your biology class?
3 a well-known scientist?
4 a fellow member of your science club?
a journalist who wrote an article in a science
a relative asking them about their job?
7 your penfriend inviting them to stay with you?
Exercise 1