There isn't necessarily a definitive answer here. The aspect that is widely touted is economic freedom, which Americans have more than any other nation on the face of the Earth. This is primarily due to the nation's free-market capitalist system and relative laissez-faire (hands-off) practice by the government when it comes to the economy.
If there was an "all of the above" choice, I would select that. America has a wide range of natural resources, particularly minerals such as coal, copper, lead, and iron. America also has wide natural gas, timber, bauxite, and uranium resources.
America is also a net exporter of technology, from corporations like Apple, Microsoft, Boeing, John Deere, etc., which are all the products of inventors.
Productivity and people can also be argued as valid choices since the nation has been and is still relatively productive when it comes to manufacturing and the like. This involves people, but when I think of people, I think of the goodness that the American people have in their hearts, always stepping up to a challenge and helping those in need.
But if one choice was needed here, I would pick Inventors, since they established the United States as a leader in technology and innovation for decades. This established the market here, bringing international and domestic consumers alike, all in-line to get high-quality American goods.