Answer:
A.To geographically contain communism
Step-by-step explanation:
At that time United States and the Ally afraid that if Communism spread to Korea , eventually The rest of Asian region will fall under communism. This is why they sent military help to south Korea and stop the spread of the ideology right there.
Technically, this could also be considered as the act of assist fledgling democracies (Option D). But option D is a little bit inaccurate because diplomacy is an effort to maintain relationship without violent actions. So war cannot be considered as a diplomatic action.