Which of the following is a legacy program provided by CUPS for sending files to the printer queues on the command line?

`lpd` (line printer daemon) is the print server daemon itself, not a command for submitting print jobs.

`lpp` is not a standard Unix/Linux command for sending files to a printer queue.

`lpq` (line printer queue) is used to display the status and contents of printer queues, not to send files for printing.

The `lpr` (line printer remote) command is a traditional Unix printing utility that CUPS implements for backward compatibility. It allows users to submit print jobs from the command line to any configured CUPS printer queue, supporting various options for job submission.