Question

What is the most identifiable and secure boundary for state lines?

Answer 1

I think the answer is C, rivers.

Answer 2

The correct answer is C) rivers.

The most identifiable and secure boundary for state lines is rivers.

Physical boundaries such as rivers are identifiable boundaries that limit or marks the natural division between states. Even between countries. That is the case for instance of the Mexican "Río Bravo" or the American "Rio Grande." It is the same river with different manes that creates a natural separation in the border with Mexico.