What threat was president truman referring to in the speech above?

I found an excerpt with the above question. It also had choices.

The excerpt goes like this:
I believe that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures... If we falter in our leadership, we may endanger the peace of the world - and we shall surely endanger the welfare of our own nation. - President Harry Truman, 1947

The choices given were:

a.     Regional wars in South America

b.    The spread of communism

c.     Civil disobedience in U.S. cities

d.    The formation of the United Nations

e.     The rise of the military industrial complex

Based on the excerpt, the threat President Truman was referring to was:

B. THE SPREAD OF COMMUNISM.