The correct answer is: still.
The word “still” is used to indicate that something has not happened or been done yet, even though it was expected to have happened or been done by a particular time. In this case, the waiter has not brought the coffee yet, even though the customer has been waiting for an hour.
The other options are not as appropriate in this context. The word “yet” can also be used to indicate that something has not happened or been done yet, but it is more likely to be used when there is a possibility that it will happen or be done in the future. In this case, it is not clear whether the waiter will bring the coffee at all, so “yet” is not the best choice. The word “up” is used to indicate that something has been completed or finished. In this case, the waiter has not completed the task of bringing the coffee, so “up” is not the best choice. The word “till” is used to indicate a point in time until which something will happen or be done. In this case, it is not clear when the waiter will bring the coffee, so “till” is not the best choice.