When a user calls in to report an issue, a technician submits a ticket on the user's behalf. Which of the following practices should the technician use to make sure the ticket is associated with the correct user?

A. Have the user provide a callback phone number to be added to the ticket.

B. Assign the ticket to the department's power user.

C. Register the ticket with a unique user identifier.

D. Provide the user with a unique ticket number that can be referenced on subsequent calls.