The correct answer is C): manufacturing.
The U.S is the world leader in manufacturing. The main explanation behind this is that the U.S make a lot of profit by selling the goods they produced in the country at a considerably higher price than the price it means producing them. In accordance with this, the level of productivity has increased enormously this is due to the aid of efficient computers and machinery.
The main products sold by this country are high-tech ones and/or high-value products which require complete professional knowledge.