(A) the end of the Western frontier
(B) a belief in spreading Western culture
(C) a need for new markets
(D) ** the decreasing population in the United States
EXPLANATIONS BELOW
Concept note-1: -The most important argument in favor of US imperialism was the financial gain to be had by giving American businesses access to colonial resources and markets.
Concept note-2: -The goal of imperialism is to acquire resources, often through exploitation and force. Motives for imperialism include economic, cultural, political, moral, and exploratory control.