The best description of mercantilism from the options provided is:
an economic system based on the idea that the world only has so much wealth available, and countries must take as much as they can so that other countries do not get it.
This definition captures the core idea of mercantilism, which emphasizes the competitive nature of nations in accumulating wealth and resources, often leading to the belief that a nation's wealth is measured by its stock of precious metals and that one nation's gain is another's loss.