The answer is (b): the area around your foot becomes dryer, as seen by comparing the photographs below and viewing an mpeg video by clicking your mouse on the photograph at the right.
Notice that when pressure is placed on the foot, simulating walking, the area around the foot becomes lighter and dryer. Just in case it is hard to see, try clicking your mouse here to see the experiment done on a beach in the Carribbean.
This phenomenon is known as "dilatancy." When water washes up on the beach and then washes back, it causes some turbulence in the sand particles near the surface of the beach. As the water leaves, the sand particles pack together with very little space between them - in fact, so little space that the water is pushed out and the surface tension of the water excludes water from flowing back into many of these cracks. When your foot applies pressure to the sand, it opens up these little cracks so that water can flow into them, with the result that the sand on the surface around your foot actually becomes dryer, as seen.
