During the 19th century, the united states ability to remain isolated from european affairs was largely due to its
a. lack of cultural identity.
b. economic instability.
c. political instability.
d. geographic location

The best answer is the geographic location:  It was separated from Europe by a body of water (the Atlantic Ocean)  that was not very easy or cheap to cross, and this provided a natural "barrier:

The other options would rather be obstacles to isolationism, not reasons that made it possible.

Akajhnanananjjijii