Final answer:
The states required federal government legislation to settle new territories west of the Appalachian mountains after the American Revolutionary War. The Northwest Ordinance of 1787 was a crucial piece of such legislation guiding the process of admitting new states and dealing with the rights of Native American tribes during westward expansion.
Step-by-step explanation:
After the American Revolutionary War, the process for states to settle new territories west of the Appalachian mountains involved the need for federal government legislation. Westward expansion patterns after the Civil War differed from that of earlier territorial acquisitions, which were generally based on settlers moving into adjacent U.S territories.
The Constitution set up a system where new states could apply for admission to join the Union, but existing states could not simply split or combine without formal consent from both the states involved and Congress. This was exemplified by the case of West Virginia during the Civil War. Moreover, the Northwest Ordinance of 1787 outlined a blueprint for nation expansion and the treatment of Native American tribes, indicating a form of regulated expansion theoretically honoring the rights of the Indigenous peoples. Therefore, the correct answer to the question is letter (d) 'The federal government needed to pass legislation allowing for the westward expansion.'