The correct answer is D) arms race.
The danger Kennedy is referring in this passage is to the arms race.
During John F. Kennedy inaugural speech, he referred to the danger of the arms race between the Soviet Union and the United States. The Cold War had created many tensions between the two superpowers and the arms race was an issue for the Kennedy administration. In the passage, we can read: "...for only when our arms are sufficient beyond doubt can we be certain beyond doubt that they will never be employed."
So the danger Kennedy is referring in this passage is to the arms race.