The refrigerant used in small tonnage commercial machines (hermetically sealed units) is

R-12 (Dichlorodifluoromethane) has been traditionally used in small tonnage commercial machines and hermetically sealed refrigeration units such as domestic refrigerators and small air-conditioning systems.

It is preferred because it is non-toxic, non-flammable, chemically stable, and has good thermodynamic properties, making it highly suitable for sealed systems where maintenance is minimal.

Other options like Ammonia are toxic, Carbon dioxide operates at very high pressures, and Sulphur dioxide is corrosive and toxic, which makes them unsuitable for small hermetically sealed units.

Although R-12 is now phased out due to its ozone depletion potential, it remains the correct theoretical answer for traditional refrigeration systems in examination contexts.