Which of the following is the best choice for lengthy or complex private messages?

A) Instant messaging
B) Email
C) A secure channel
D) Acronyms
E) Mobile blogging

Email provides a better platform for lengthy and complex private messages as it allows for more detailed and organized content, attachments, and the ability to reference previous messages. Instant messaging may not be suitable due to character or time limitations, and a secure channel may not be necessary for all messages. Acronyms and mobile blogging are not appropriate for lengthy and complex private messages.