The correct answer is: A. Surface moisture.
Bulking of sand is the increase in volume of sand when it is mixed with water. This is caused by the adhesion of water molecules to the sand grains, which causes them to swell and separate from each other. The amount of bulking that occurs depends on the amount of water present, the type of sand, and the degree of compaction.
Air voids are spaces between the sand grains that are filled with air. They do not contribute to bulking, as they do not affect the volume of the sand.
Viscosity is a measure of the resistance of a fluid to flow. It does not affect bulking, as sand is a solid and not a fluid.
Clay contents can affect bulking, as clay particles are smaller than sand particles and can fill in the spaces between the sand grains. However, the effect of clay contents is usually small, and bulking is most commonly caused by surface moisture.