The correct answer is D) Cuba's proximity to the United States made it possible for large-scale missile attacks.
Based on this map, what made Cuba such a serious threat to the United States during the Cold War was that Cuba's proximity to the United States made it possible for large-scale missile attacks.
The Florida peninsula and the tip of the island of Cuba are separated by only a hundred miles. In the times of the Cold War, this proximity represented a great risk for the United States because the Soviet Union had a total presence in Communist Cuba. Indeed, the planet was on the brink of another world war with the Cuban Missile Crisis of October 1962.