Which of the following materials is most porous? shale clay unsorted sand and rock loosely packed gravel

The correct answer is loosely packed gravel.

The water absorbing capacity of a substance is known as its porosity. The water absorbing capacity of loosely packed gravel is very high. It can absorb a lot of water and because of this quality it is used in aquifers.This is the property of loosely packed gravel that it can hold upto 25% of water which means its porosity is 25%.