Why is the study of doabs useful for understanding river systems?

Correct answer: They show alluvial land between two rivers

Direct answer: Option C. A doab is a tract of land lying between two rivers; the word can be understood as “two waters.” Rivers often deposit layers of fine sediment called alluvium on such land. Consequently, many doabs are fertile and useful for agriculture, and their location helps students understand how rivers, tributaries and drainage basins are arranged. Option C correctly identifies alluvial land between two rivers. Option A is wrong because land between two seas is not the definition of a doab. Option B describes a mountain route or pass, not river-created land. Option D refers to coral between islands, a marine feature unrelated to doabs. The Ganga–Yamuna Doab is the familiar Indian example: it lies between the Ganga and Yamuna rivers. Memory cue: doab = do, two; ab, water—land between two rivers.