What were the foundations of the American spirit of free enterprise?

It seems likely that your textbook gives an answer that
you are supposed to come up with.  I say this because this is a
very subjective question and there is no real objective way
to answer it.  So I really suggest that you check your book to see what it
says.

One can argue that the foundation of the American
spirit of free enterprise was the Protestant work ethic that Max Weber talked about. 
This idea holds that Protestants were driven by the idea that work was righteous for its
own sake.  People should work as hard as possible at all times rather than just doing
enough to get by.  This attitude, it can be argued, caused Americans to be very
capitalistic and to value free enterprise.

One can also
argue that it was the lack of class structure that made the US value free enterprise. 
Since in America all people were equal and all people could get ahead, there was a real
incentive to work hard and to be entrepreneurial.  This helped cause Americans to
believe in free enterprise.

Either of these is a plausible
answer and other answers are possible.  I once again recommend that you check your book
for what it says is the right answer.