If your ticket was issued for Business then it is Business. The letter D usually refers to the tariff being given at the time of purchase.

If you are flying Qatar Airways then you are entitled to go to the Premium Terminal.

If your business class is with any other airline then you must show up for check in at departures terminal B.

Regards!!

PS: if you still have questions regarding your ticket, tariff, and class refer back to the Travel Agent.