What command is used to send the email content after the DATA command?

The correct command to signal the end of the email content after the DATA command is a single line with a period (.). This is a fundamental aspect of the SMTP protocol used for email transmission.

When the sender issues the DATA command, the server is prepared to receive the actual message content. After the message has been composed and sent line by line, the sender needs to indicate that they have finished transmitting the body of the email. By using a lone period on a line by itself, the sender communicates to the server that the email content is complete and that it can now process the message. This is a defined part of the SMTP protocol, ensuring a clear demarcation between the email body and the end of the transmission.

The other options involve aspects unrelated to the specific requirement of concluding the message content. They do not contribute to the SMTP protocol's methodology of finalizing the email transmission and are therefore not relevant to the task at hand.