He most common economic system in the world is a

closed economy.
command economy.
market economy.
traditional economy.

The answer is: market economy.

Market economy refers to an economic system in which production, distribution, and investment is dictated by the forces of supply and demand (in which the two are correlated, so that, in an ideal sense, the greater the demand, the less the supply, and vice versa, but always aiming at balance). In principle, a market economy would allow for productive forces (more or less regulated by government), through competition, to balance themselves so that capital can be distributed amongst these forces.  

the roman empire began to fall