Which of the following countries would have the most difficulty responding to rapid population growth?

Italy has a relatively stable population and is currently experiencing low birth rates, which leads to a gradual population decline. The country has well-established infrastructure and social services that can adequately manage its population dynamics. Thus, Italy would not face significant difficulties responding to any population changes.

Similar to Italy, Spain has a low birth rate and a population that is stabilizing. While Spain has its own economic challenges, its population growth is not rapid, allowing it to maintain effective governance and social services. Consequently, Spain would not struggle significantly with rapid population growth.

The United States, while facing its own population challenges, has a robust economy, advanced infrastructure, and a diverse range of resources to effectively manage population growth. The capacity to adapt to demographic changes is comparatively higher in the U.S., making it less susceptible to difficulties arising from rapid population increases.