Final answer:
The correct phrase to complete the diagram about Atlantic South America's geography is 'Large river basins', as this region is dominated by the Amazon Basin and its extensive river systems.
Step-by-step explanation:
The phrase that best completes the diagram linking the geography of Atlantic South America with its other physical characteristics is 'Large river basins'.
South America, particularly the Atlantic portion, is well-known for its vast and intricate river systems, which include the sprawling Amazon Basin. The Amazon Basin is characterized by the massive Amazon River, which is one of the longest and most voluminous rivers in the world. This region features a type A climate and hosts tropical wet forests with high species diversity. The Amazon River and its tributaries cover a significant portion of the continent, shaping the natural landscapes and ecosystems.
Options such as coniferous forests and narrow isthmuses are less pertinent to the characteristics of Atlantic South America in comparison to the presence of large river basins, which are crucial to the region's geography. Therefore, option D, 'Large river basins', accurately represents a major physical feature of Atlantic South America alongside other features like huge rain forests and lengthy coastlines.