Which should be written in a formal style?

A. Personal e-mails

B. friendly letters

C. Movie reviews

D. Work document

work documents.

Explanation:

... The same happens for letters and emails that you may write to your friends. In relation to movie reviews, while they are often somewhat formal, these are also personal portrayals of opinion.

D. Work Document

Explanation:

Because a work document is for a serious matter, you would want to write it in a formal manner, and not in a casual manner