The most common refrigerants used in domestic refrigerators are Freon (chlorofluorocarbons or CFCs) and Ammonia. These substances are specially designed for cooling purposes in refrigeration systems. Freon was widely used for many years because it has excellent thermodynamic properties - it evaporates at low temperatures, absorbing heat from the refrigerator interior, and then condenses back to liquid when compressed, releasing heat outside. Ammonia is another effective refrigerant that works similarly, though its more commonly used in industrial systems. Both substances are chosen because they can easily change between liquid and gas states at convenient temperatures for refrigeration. However, Freon has been phased out in many countries due to its harmful effects on the ozone layer, and newer environmentally friendly refrigerants are being developed. The other options - Neon, Spirit, and Methane - are not suitable as refrigerants because they don't have the right thermodynamic properties for efficient cooling in household refrigerators. Modern refrigerators now often use hydrofluorocarbons (HFCs) as safer alternatives to traditional Freon, though they still follow the same basic principle of heat absorption through phase change.