Final answer:
The Great Plains is the area in the U.S. that is part of the temperate grassland biome, characterized by prairies with significant temperature fluctuations and a dominance of grasses.
Step-by-step explanation:
The area of the U.S. that is part of the temperate grassland biome is the Great Plains. Temperate grasslands, also known as prairies in North America, are characterized by a semi-arid climate and relatively flat landscapes dominated by grasses rather than trees. This biome experiences significant seasonal temperature variations, with hot summers and cold winters. The Great Plains extend across the central part of the continent, serving as an expansive area where this biome thrives. In contrast, the other options listed, such as Alaska, Florida, and the Rocky Mountains, are associated with different biomes like the tundra, subtropical, and montane ecosystems, respectively.