Threshing is the process of loosening the edible part of grain from the husks and straw to which it is attached. It is the step in grain preparation after reaping and before winnowing, which separates the grain from the chaff(dust). 

When rice is washed with water, the dust particles get mixed with water and get separated. Since dust along with water is removed, so the process is decantation.

Sedimentation is just letting it settle in a liquid medium and the impurities get sedimented. It is not suitable here because it will destroy the quality of rice if they are kept for a long time. 

Also, Sieving is defined as a method in which two or more components of different sizes are separated from a mixture on the basis of the difference in their sizes. So, the removal of dust particles from rice is difficult with this method as they both almost have the same size.