Which of the following is a direct service "drop-in" substitute for R-22?

The correct choice is R-410A because it is specifically designed to be used as a drop-in refrigerant for R-22 systems with some modifications. While both R-22 and R-410A are used in residential and commercial air conditioning systems, R-410A operates at a higher pressure than R-22, making it necessary to use equipment that is rated for this higher pressure. However, it is important to note that while R-410A can be fitted into systems that were originally using R-22, the proper handling and system modifications must be taken into account.

R-134a and R-141b are not appropriate substitutes for R-22 in terms of directly replacing R-22 in existing systems. R-134a has different properties that make it unsuitable for use in direct replacement applications for R-22 systems, and R-141b is typically used in other applications, such as foam-blowing agents and has been phased out due to its high ozone depletion potential. Therefore, R-410A is the recognized alternative for a direct service substitution for R-22 in the context of existing air conditioning systems.