Which should be written in a formal style ?

a. personal e-mails
b. friendly letters
c. movie reviews
d. work document

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

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 .

the unique function that the video specifically ascribes to sins near tunnels is "prohibiting certain cargo". tunnels have specific control signs such as minimum and maximum speed, the appropriate use of headlight and prohibited cargo. for your own safety it is important you understand the signs when approaching a tunnel.