Essential business letter elements include the ____.

A) date line and inside address
B) message
C) signature block
D) all of the above

All three elements, date line and inside address, message, and signature block, are necessary components of a complete and professional business letter. The date line and inside address provide important information about the sender and intended recipient of the letter, while the message contains the main content of the communication. The signature block at the end of the letter ensures that the sender's identity is clear and recognizable, and also serves as a formal closing to the communication. All of these elements work together to create a professional and effective business letter.