The correct answer is A. Western United States.
In all of the other mentioned regions the borders are usually made by the lines and forms of geographic entities, like mountains, rivers, lakes etc. In Western Unites States it's a different situation, the borders are very often as straight lines, this is mainly due to the fact because they are confederate states that are part of the same country, so there's no situation where there's disputes over pieces of land, and those borders are created by the countries authorities.
