The correct answer is D : United States
All of the other countries on the list have great species diversity, especially Cosa Rica due to it's closeness to the equator and therefore its tropical climate perfectly suitable for wildlife thrive.
But, the United States' territory covers the areas with the mild climate, forests, deserts, canyons and species-rich coastal regions as well as the Hawaii islands with unique species diversity and the Alaskan area with the arctic habitat.