The best example of a boundary is a. a river that divides two b. an underground tunnel c. an open space of land with mountains and rivers d. a defined territory with an established government  

The answer is going to be a. hope that helped

It should be A. a river that divides two 
because if it were an underground tunnel, people wouldn't see it and walk across it. If it was an open land with mountains of rivers, then people would also walk across it. If it was a territory established by a government, people could still get through it.
I hope I helped
Please make me brainliest

Answersource rock;

I’m guessing probably a