If there are multiple recipients involved and you want to reply to just the person who has sent the ticket, click Reply (from inside the editor).
Within the editor, compose your response for the ticket. When you're done composing, click the Send button at the top of your editor window. This will complete your action of responding to a ticket. You can also click Send & Close to send a reply and close the ticket simultaneously.
Your recent response will now be listed just below the title of the ticket. You can view the date and the time at which you sent the reply.