We have to understand what exactly "heart of the question" is to be able to use process of elimination to discover what the best answer will be. If we break it down, we can determine what it means. The heart is at the center of a human body and is necessary for all functions. If we're looking at the heart of the question, then this means we're deciding what's the most important thing or the central thing that we are looking at.
Going through your choices, decision of the President doesn't seem likely because it doesn't sound like President Kennedy is talking about any decision he has to make.
Reason for believing could work, but the way Kennedy has worded this part of his speech doesn't imply a reason for believing. He is asking <em>if</em> people believe.
Theme of the problem is a great answer choice because it fits well with what Kennedy is asking here. He is asking the American people if they believe all people should be equal. This is the best choice.
While center of the nation is also a good choice, it is not the best choice here because while this issue might be central to the nation, that is not what Kennedy is discussing or implying with this part of his speech. There is not mention of anything dealing with this.
