What is a common source of ignition associated with propane?

The correct choice is the electrical switch, as it can serve as a common source of ignition associated with propane. Propane is a highly flammable gas, and any electrical devices that create a spark or heat, such as switches, can ignite propane if there is a leak or accumulation of the gas in the surrounding area. Proper precautions must always be taken when working with propane to prevent ignition sources from coming into contact with the gas.

The other options do not present a viable ignition source. Sunlight, while capable of heating objects, does not directly ignite propane without another ignition source. Fresh air, on the contrary, helps to dilute propane concentrations in the air, reducing the risk of ignition. Water is non-flammable and does not pose an ignition risk to propane at all. Understanding the specific ignition sources associated with propane is critical for safety in environments where propane is used or stored.