Which of the following is a typical ignition source for propane?

A typical ignition source for propane is indeed cigarettes. Propane is a highly flammable gas and can ignite upon contact with any open flame or a spark. Cigarettes, when lit, produce a flame that can easily ignite propane vapors if they are present in the vicinity. This makes them a significant potential ignition source.

In contrast, while air conditioning units, plastic materials, and water heaters can also pose risks in specific contexts, they are not as direct or common as cigarettes when it comes to igniting propane. Air conditioning units generally operate without producing flames, plastic materials typically do not ignite propane directly, and water heaters, while they may involve flame or heat, are not routinely considered primary ignition sources for propane compared to cigarettes.