Which river separates the central and western mountain ranges in Colombia and is a main tributary of the Magdalena?

The Cauca River is the correct answer because it acts as a significant geographical feature in Colombia, delineating the central and western mountain ranges. It is a crucial tributary of the Magdalena River, which is one of the most important rivers in Colombia. The Cauca River flows northward through the valleys between these mountain ranges and directly contributes to the flow of the Magdalena River, which further enhances its significance in the region's hydrology.

The other rivers listed do not fulfill this specific role in the geographical context of Colombia. The Atrato River, for instance, is primarily located in the northwest of the country and is not a tributary of the Magdalena River. The Magdalena River is significant on its own but does not separate the mountain ranges. The Orinoco River, while important in South America, flows through a different geographic area and does not relate to the mountain ranges or the Magdalena River. Thus, the relevance of the Cauca River in separating the mountain ranges and being a main tributary makes it the best choice for this question.