Agricultural usefulness of doabs is most linked with which physical factor?

Correct answer: Alluvial soil between two rivers

The direct answer is A, alluvial soil between two rivers. A doab is the land lying between two rivers; the word can be understood as “two waters.” Rivers deposit fine silt, sand and clay on their floodplains, forming fertile alluvial soil. With water availability and fertile deposits, doabs are especially useful for farming. The reasoning is: two rivers enclose the area; repeated deposition creates alluvium; fertile soil and water support crops. Option A is correct. Option B, a thick layer of sea salt, would make soil saline and generally harm many crops; a doab is not defined by the sea. Option C, permanent snow cover, occurs in very cold high mountains and prevents ordinary cultivation, so it does not describe doabs. Option D, uplift of coral rocks, relates to some coastal or island landforms, not the river-deposited plains between two rivers. The Ganga–Yamuna Doab is the familiar Indian example, located between the Ganga and Yamuna. Memory cue: doab means do (two) plus ab (water), therefore land between two rivers.