Which of the following commands should be used to print a listing of emails in the system's mail queue?

`sendmail -l` is not a standard or correct option to list the mail queue; `-bp` or `mailq` is typically used with sendmail.

`lpq` is a command used to inspect the print queue, not the email queue.

The `mailq` command is a standard utility on Unix-like systems, often a symlink to or feature of an MTA like `sendmail` or `postfix`. It provides a summary of messages currently in the mail queue, showing details such as message IDs, senders, recipients, and current status, indicating emails waiting to be processed or delivered.

`mlq` is not a recognized or standard command for listing the mail queue.