Final answer:
The Coastal Plain region is characterized by its flat or gently sloping terrain near a coastline, as seen in the descriptions provided. To identify it on a map with letters, one should look for the area with less steep contours near the coast. Without seeing Map 3, we cannot definitively match the Coastal Plain to a letter, but can infer that it is likely the less elevated region near the ocean.
Step-by-step explanation:
To determine which letter identifies the Coastal Plain region, we need to reference the descriptions provided in relation to Map 3. The Coastal Plain is described as a region along the Atlantic Ocean and Gulf of Mexico, which features shallow marine sedimentary environments and the accumulation of sediment since the rifting of Pangaea. It's characterized by an extensive coastal plain, river basins, wetlands, and a history intertwined with plantation agriculture.
The details provided about plantation agriculture, landscape, and history such as in the Carolinas (5.2) and the Tropical Plantation Region give us clues about the geographical features of the Coastal Plain. These clues lead to the determination that the Coastal Plain is likely the flatter area bordering the coast, as opposed to the areas with steeper relief or higher elevations inland or areas identified as mountainous.