What causes static electricity to build up on materials?

Static electricity builds up on materials primarily due to the accumulation of electrical charges on their surfaces. When two different materials come into contact and are then separated, electrons can be transferred from one material to the other. This transfer leaves one surface positively charged and the other negatively charged, leading to an imbalance of electrical charges. As these charges collect on the surface of materials, static electricity is generated.

This phenomenon is influenced by factors such as the type of materials interacting, the conditions of the environment (like humidity), and whether there is sufficient contact and friction between them. It's important to understand that static electricity is fundamentally about the imbalance of charges, which occurs on the surfaces of materials, making this the correct answer in the context of the question.

The other choices, while they may relate to the environment or related phenomena, do not directly explain the mechanism of static electricity accumulation on materials themselves. For instance, moist air generally reduces static electricity buildup due to the presence of water molecules, which helps dissipate charges.