The correct answer is D. still.
The word “still” is used to indicate that something has not happened or changed, even though it should have or could have. In this case, the waiter has not brought the coffee, even though the customer has been waiting for an hour.
The other options are not correct because they do not convey the same meaning. The word “till” means “until,” so it would not be used to indicate that something has not happened. The word “up” is used to indicate a position or direction, so it would not be used to indicate that something has not happened. The word “yet” is used to indicate that something has not happened but is expected to happen in the future, so it would not be used to indicate that something has not happened in the past.