Which function of a river mainly carries material from mountains to plains?

Correct answer: Transportation

A river performs several geomorphic functions, including erosion, transportation, and deposition. Transportation is the movement of material by flowing water from one place to another. In mountainous areas, rivers can pick up rock fragments, gravel, sand, and silt and carry them downstream toward lower land and plains. Deposition happens later when the river loses energy and drops some of that material, so it is different from carrying it.

The question asks which function mainly moves material from mountains to plains. Option B, Transportation, is correct because it names the carrying and movement process. Deposition means laying material down, condensation is a change of water vapour into liquid, and explosion is unrelated to normal river work. A river may both transport and later deposit sediment, but the movement from the mountain region to the plains is specifically transportation. Thus the supplied answer B is accurate.