The correct answer is B. Subject.
The subject line is a brief description of the email’s contents. It is used to help the recipient decide whether to open the email and to identify the email when they are looking through their inbox.
The To line specifies the email address of the recipient. The CC line specifies the email addresses of people who should receive a copy of the email, but who are not the primary recipients. The BCC line specifies the email addresses of people who should receive a copy of the email, but who should not be revealed to the primary recipients.
The Contents of the email are the body of the message. This is where the sender provides information or asks a question.