What part of an email should briefly describe its purpose?

The subject line of an email serves as a concise summary of its purpose, allowing the recipient to quickly grasp the main topic before opening the message. It should be clear and to the point, often containing keywords related to the content of the email. A well-crafted subject line can help prioritize the email in a crowded inbox and can prompt the recipient to open it more readily.

The body of the email contains the detailed information and the main message, while the signature provides the sender's contact information and credentials. Attachments refer to files that may be included with the email for additional information or resources, but they do not describe the email's purpose. Hence, the subject line is the correct answer as it directly addresses the need for a brief description of the email's intent.