D/s is what you write (duration of status) since the F visa doesn't have an expiration date and is valid for a long as the student is complying with its terms.
The information offered is general in nature and not meant to be relied upon as legal advice. Please consult an attorney prior to making legal decisions. Visit us at www.tunitskylaw.com. Contact us at 713.335.5505 or email at email@example.com. Veronica Tunitsky offers in-person, as well as telephone and email consultations.
I agree with Ms. Tunitsky. There may also be other benefits related to those who have entered the U.S. with duration of status. For more information, contact an experienced immigration attorney.
This is general information, not legal advice, and does not create an attorney client relationship.