Final Answer:
The main reason the colonists assembled for the First Continental Congress was B) to discuss their grievances with British policies.
Step-by-step explanation:
The correct option is b. to discuss their grievances with British policies.
The question directly asks for the main reason for the assembly of the First Continental Congress. The colonists convened in 1774 in response to their dissatisfaction with British policies and to address the issues they had with the British government's actions.
Explanation for Option a (to demand the right to settle west of the Appalachian Mountains):
This option is not true. The issue of settling west of the Appalachian Mountains was more relevant to events before the American Revolution, such as the Proclamation of 1763, which restricted westward expansion.
Explanation for Option c (to decide how much tax they wanted to impose on British goods):
This option is not accurate. The primary concern of the First Continental Congress was not about imposing taxes but rather about addressing broader grievances and asserting their rights.
Explanation for Option d (to work out an arrangement with the Native Americans):
This option is not correct. The focus of the First Continental Congress was mainly on addressing colonial grievances with British policies, and discussions about Native American arrangements were not the primary purpose.
In summary, the key reason for the colonists' assembly at the First Continental Congress was to address and discuss their grievances with British policies.