The correct answer here has to got to be A -- a favorable balance of exports over imports. In other words, you get rich by exporting more to other countries than you import from them.
This was the major economic idea back in most of the 18th century. And it is one reason why countries took colonies. They wanted to have colonies where they could buy things cheaply (things like raw materials) and where they would be the only ones who could sell manufactured goods. This would give them a favorable balance of trade and make them rich.