Picture a dry sponge dropped into water compared with a damp cloth that only holds droplets on its outer surface. In the sponge, water molecules cross the outer boundary and spread evenly through the entire three-dimensional body of the material, which is the whole idea behind absorption. In adsorption, on the other hand, the sorbate molecules stay stuck on the outer surface layer and never really travel into the interior, so the concentration is much higher right at the surface than deep inside. Since the question describes a substance spreading evenly through the whole bulk volume of the host material, this is absorption, not a surface effect like adsorption, chemisorption, or desorption. So the correct choice is option (C).