To save a mail in a separate file while working with the mail command, you can use the command "s".
Option A) s - This option is correct because the "s" command is used to save a mail in a separate file. When you are composing or replying to a mail using the mail command, you can use the "s" command to save the mail in a file.
Option B) r - This option is incorrect. The "r" command in the mail command is used to reply to a mail.
Option C) w - This option is incorrect. The "w" command in the mail command is used to write the mail to a file.
Option D) d - This option is incorrect. The "d" command in the mail command is used to delete a mail.
The correct answer is A) s because it is the command used to save a mail in a separate file.