Refrigerators work by circulating a refrigerant that undergoes phase changes (evaporation and condensation) to absorb and release heat. Historically, Chlorofluorocarbons (CFCs) like Freon-12 (Dichlorodifluoromethane, CCl2F2) were widely used as refrigerants in refrigerators and air conditioners due to their properties (non-toxic, non-flammable, stable). However, CFCs were found to be highly damaging to the ozone layer. Due to international agreements like the Montreal Protocol, the use of CFCs has been largely phased out, and they have been replaced by Hydrochlorofluorocarbons (HCFCs), Hydrofluorocarbons (HFCs), hydrocarbons (like isobutane, propane), ammonia, and other substances. While modern refrigerators may use different refrigerants (often hydrocarbons or HFCs), the question asks what is present. Among the options provided, CFCs are historically the most well-known refrigerant associated with refrigerators. Carbon dioxide (R-744) is used as a refrigerant in some systems, but CFCs were ubiquitous for decades. Methane is a hydrocarbon, some hydrocarbons are used, but not methane directly in most consumer refrigerators. Helium is used in cryogenics, not standard refrigerators. Given the options, Chlorofluorocarbon is the most appropriate answer representing a substance known to be present in refrigerators.