It's all in the process of elimination.
Choice A: It is a plea to support religious freedom. (Incorrect)
The excerpt never said anything about <em>religious </em>freedom. It only said, "My fellow citizens of the world: ask not what America will do for you, but what together we can do for the freedom of man," in which this was the only reference to "freedom."
Choice B: It is a command to end selfish behavior. (Incorrect)
Sure, Kennedy, throughout his administration, was as selfless as could be (supposedly) but that was not the main point of his inaugural speech.
He states, "And so, my fellow Americans: ask not what your country can do for you—ask what you can do for your country."
Even though he is technically telling his audience not to be "selfish," again, it is not the main idea and he wasn't "commanding" them.
Choice D: It is a warning of the dangers of inaction. (Incorrect)
If old Johnny-boy wanted to warn his audience he would have done so instead of coming of with this hippie approach. He rants on about "good deeds" and "love" which does not correspond with Choice D.
Choice C: It is a call to support the greater good. (CORRECT)
Kennedy states, "My fellow citizens of the world: ask not what America will do for you, but what together we can do for the freedom of man."
He also says, "Finally, whether you are citizens of America or citizens of the world, ask of us here the same high standards of strength and sacrifice which we ask of you."
John F Kennedy is not "pleading," is not "commanding," is not "warning," but "calling" human beings to support the greater good through "strength and sacrifice" for the "freedom of man."
Therefore, the answer is Choice C.