Final answer:
Imperialism refers to a policy by which one nation controls another country or territory, usually for political, social, and economic dominance.
Step-by-step explanation:
The policy or action by which one nation controls another country or territory is referred to as imperialism. This involves a stronger nation seeking to dominate other countries politically, socially, and economically. Imperialistic powers often use military conquest to secure control over foreign territories, establish markets, and prevent other powers from encroaching on their colonies. An example of this was seen when Great Britain competed with Russia for control over Afghanistan to protect its interests in India. Therefore, the correct answer is A: Imperialism.