Of the following materials, which is the most porous?

The most porous materials on those materials you enumerated is sandstone. Sandstone contain sand sized grains that can easily be distinguished with a petrographic microscope. Most of the world's oil and natural gas is found in sands or sandstone because of their generally high porosity. Mike from-a href="http://planetmarblegranite.com" rel="nofollow"http://planetmarblegranite.com/a