The United States is a mixed economy.
You can eliminate the other possibilities with facts;
A market economy is an economy with no government involvement, but that is not the case in America due to business loans, government-located health inspectors and police, etc.
A command economy is an economy with total government involvement. This isn’t the case since a person has freedom to choose or create a business to work at.
A traditional economy is one with things such as tribes and small social groups. (It’s been a while since I’ve learned about this one, so forgive me if my memory’s a bit rusty.) This isn’t the case for obvious reason.
And then, by both process of elimination and the fact that the answer makes sense, we can conclude D is the answer.