What are the different ways a difference of potential can be established?

The correct answer highlights multiple phenomena that can establish a difference of potential, known as voltage. Friction can produce static electricity when two materials are rubbed together, causing an imbalance of electric charges. Thermal action refers to the generation of voltage through temperature differences, commonly observed in thermoelectric generators. Magnetic induction occurs when a changing magnetic field creates a voltage in a conductor, a principle used in generators and transformers. Chemical action involves the electrochemical processes in batteries and fuel cells, where chemical reactions produce a flow of electrons, leading to a potential difference. Including light as a source recognizes phenomena like the photoelectric effect, where photons can free electrons from a material, thereby creating a potential difference.

Other options do not encompass the complete range of methods for generating a potential difference. For example, while they may include relevant phenomena, they omit essential processes like magnetic induction or may introduce unrelated concepts, such as sound and pressure, which do not inherently create voltage. Electromagnetic waves themselves do not directly establish a potential difference; rather, they are a way energy can be transmitted through space. Thus, the selected answer comprehensively captures the various ways voltage can be established through different physical processes.