Which mountain range is primarily located in the eastern United States and is one of the oldest in North America?

The Appalachian Mountains are the correct answer because they are primarily located in the eastern United States and are renowned for being one of the oldest mountain ranges in North America. This range stretches from Newfoundland in Canada all the way down to Alabama, covering several states along the eastern side of the country. The mountains were formed over 480 million years ago during periods of tectonic activity, leading to their significant geological and ecological diversity.

The Rocky Mountains, although they are a prominent mountain range in the U.S., are located in the western part of the country and were formed more recently, mainly due to tectonic plate movements. The Sierra Nevada Mountains and the Cascade Mountains are also located in the western United States, with the Sierra Nevada primarily in California and the Cascades in Washington and Oregon, and both have younger geological formations compared to the Appalachians.

This context highlights why the Appalachian Mountains stand out as the correct choice for this question concerning age and location within the United States.