The correct answer is - B. increase the amount of arable land.
With the usage of the terraces people are able to use much more land for agriculture, and this is because the places were the terraces are made are mostly very steep hillsides and mountainsides where agriculture will be impossible in their initial form. By making thew terraces, people flatten parts of the steep sides and they look like very big circular steps. The terraces are mostly used in East and Southeast Asia for the production of rye for the huge populations that live there and the huge demand for the crop, so any piece of land that can be used is used.