Final answer:
R-134a is the approved "drop-in" replacement for R-12, offering similar thermodynamic properties and being an environmentally friendly option.
Step-by-step explanation:
The approved "drop-in" replacement for R-12 is R-134a. It is a hydrofluorocarbon (HFC) and does not contain chlorine, making it an environmentally friendly option. R-134a has similar thermodynamic properties to R-12, making it a suitable replacement in refrigeration and air conditioning systems.