Answer:
The British act that prohibited settlement west of the Appalachian Mountains was the Royal Proclamation of 1763.
Step-by-step explanation:
The Royal Proclamation of 1763 was issued on 7 October 1763 by British King George III. It divided the British territories newly acquired in the Treaty of Paris in 1763 into North America and regulated the trade, land acquisition and resettlement of European settlers beyond the proclaimed proclamation line. The proclamation line was a temporary border between the colonies and the Indian Reserve. It was then largely marked by the Appalachians. The proclamation was supposed to stabilize relations with the Indians of North America.