The correct answer is premium.
A discount is a reduction in the price of something. A premium is an additional amount of money that is paid for something, especially in order to get insurance or to join a club.
Interest is a fee paid for the use of money. Profit is the amount of money that is left over after all of the costs of doing business have been paid. Concession is a reduction in the price of something, especially one that is given to a particular group of people.
In the context of the question, the antonym of discount is premium. This is because a premium is an additional amount of money that is paid for something, while a discount is a reduction in the price of something.