Final answer:
The feature that is not commonly used as a physical separator for states is political treaties. Natural features such as rivers, mountains, and deserts often serve as borders, unlike political treaties, which are formal agreements.
Step-by-step explanation:
A feature of the physical environment commonly used to separate states includes all but which of the following? Rivers, mountains, deserts, or political treaties? The correct answer here is political treaties. Natural geographic features like rivers, mountains, and deserts are often used as natural borders between political units such as cities, states, or countries. For instance, the Rio Grande serves as a portion of the border between the United States and Mexico, while the Himalayan Mountains form a boundary separating India and China. These natural formations can be easily identified on a map and are historically significant for their role in defining geopolitical spaces.
In contrast, political treaties are not physical features of the environment but rather formal agreements between states that establish boundaries through negotiation. Thus, while they are critical to the delineation of political spaces, they are conceptually different from natural geographic features used as borders.