Final answer:
The correct 'drop-in' replacement for R-12 is HFC-134a, which is an EPA-approved hydrofluorocarbon that can be used without significant modifications to existing systems. Other refrigerants like HFC-410a and R-407c require modifications and are not considered 'drop-in' replacements.
Step-by-step explanation:
Approved 'Drop-in' Service Replacement for R-12
When it comes to replacing R-12, a chlorofluorocarbon (CFC), there are a few alternatives that have been developed due to the environmental harm caused by CFCs, which include ozone depletion. The most promising replacements include hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), specifically HFC-134a (CF3-CH₂F). HFC-134a has been widely recognized as a drop-in replacement option for R-12 in refrigeration and air-conditioning systems due to its lower ozone depletion potential and similar thermodynamic properties.
While HFC-410a and R-407c are also replacements for CFCs, they are not considered 'drop-in' replacements for R-12 because their properties and system requirements differ significantly from those of R-12. These refrigerants may require modifications to the existing equipment to function correctly, which is not in line with the 'drop-in' concept.
Therefore, the correct answer to the student's question is 1) HFC-134a, as it is an EPA-approved 'drop-in' replacement for R-12 in many systems.