The best explanation for why the Rhine-Ruhr area became a major industrial area is B. it has major iron and coal deposits.
While factors like central location (A), population (C), and flat land (D) also contribute to its industrial development, the availability of local raw materials, particularly iron and coal, played a crucial role in the area's rise as an industrial center during the 19th and 20th centuries. The proximity to these resources allowed for the growth of heavy industries, such as steel production and manufacturing.