单项选择题

It is often difficult for a man to be quite sure what tax he ought to pay to the government because it depends on so many different things: whether the man is married; how many children he has; whether he supports any relations; how much he earns; how much interest he receives; how much he has spent on his house during the year, and so on and so forth. All this makes it difficult to decide exactly how much the tax is.
There was a certain artist who was always very careful to pay the proper amount.
One year, after posting his check as usual, he began to wonder if he had paid enough, and after a lot of work, with a pencil and paper, he decided that he had not. He believed that he owed the government something.
He was just writing another check to send to the tax-collector when the postman dropped a letter into the box at the front door. Opening it, the artist was surprised to find inside it a check for five pounds from the tax-collector. The official explained that too much had been paid, and that therefore the difference was now returned to the taxpayer.
The word "proper" in the first line of the second paragraph means ______.