The correct option is:

The correct option is:

• Always requires a professional tone.

A formal email is used for professional and academic communication, which necessitates a professional tone to convey respect and seriousness. Options like "friendly and personal," "uses slang and emojis," and "grammar and accuracy are not a property" are characteristics of informal communication, not formal emails.

The final answer is $\boxed{\text{Always requires a professional tone.}}$