Which of the following is an example of a natural obstacle?

A natural obstacle is defined as a feature of the Earth's terrain that affects movement and tactics in a military context, which exists without human modification. Steep banks are a prime example of a natural obstacle because they are typically formed by geological processes and can impede movement, making them significant in planning maneuvers and positioning during combat operations. They can restrict access and visibility, forcing units to adapt their strategies accordingly.

In contrast, the other choices represent man-made obstacles. Concrete walls, barbed wire fences, and sandbags are all modifications to the environment created to serve particular functions in defense or fortification. Understanding the distinction between natural and artificial obstacles is crucial for effective navigation and tactical planning in diverse combat scenarios.