Answer:D. use abbreviations such as "u," "r," and "BRB" to keep the e-mail message shorter
Explanation:This is an informal writing which one can use when chatting to their friends but it can not be used in formal writing such as when one writes an email.
The email is usually used for formal communications and when one needs it to be short, the way that can be appropriate to achieve that will be concise and to the point .