What should be written in a formal style?

a. personal e-mails

b. friendly letters

c. movie reviews

d. work documents

Among the available options, the kind of document that often requires the most formal form of speech and objectivity is work related documents. Usually, when you talk with your friends, you don't use formal speech. 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 and tend to be on the subjective side.

Therefore, your best answer is option D.