Most porous is gravel
Porosity tells us how much water rock or soil can retain. It tells us about the spaces among the grains. Gravel is more bigger than sand or silt. Its grains are arranged in such a manner that their integral spaces are very bigger so they are more porous. Clay is the most porous but since it is not mentioned in the option so after clay comes the gravel. Usually gravel and sand has equal porosity.
