In most cycles, it undergoes phase transitions from a liquid to a gas and back again.
Many working fluids have been used for such purposes. Fluorocarbons, especially chlorofluorocarbons, became common place in the 20th century, but they are being phased out because of their ozone depletion effects.
Other common refrigerants used in various applications are ammonia, sulfur dioxide, and non-halogenated hydrocarbons such as propane.
Freon is a registered trade name of DuPont, which it uses for a number of halocarbon products.
They are stable, nonflammable, moderately toxic gases or liquids which have typically been used as refrigerants and as aerosol propellants.
Therefore, option D is correct.