when is it ok to use slang language in an email to a client?\no after one year\no never\no anytime\no after…

when is it ok to use slang language in an email to a client?\no after one year\no never\no anytime\no after one month
Answer
Brief Explanations:
Emails to clients should be professional. Slang is informal and may give a bad impression, so it's best avoided.
Answer:
B. never