Which refrigerant was commonly used in MVAC systems prior to 1994?

Prior to 1994, R-12 was the predominant refrigerant used in motor vehicle air conditioning (MVAC) systems. R-12, also known as dichlorodifluoromethane, is a chlorofluorocarbon (CFC) that, while effective as a refrigerant, was found to contribute significantly to ozone depletion in the stratosphere. As a result of environmental concerns and the implementation of regulations, such as the Montreal Protocol, the production and use of R-12 were phased out, leading to a shift to alternative refrigerants like R-134a.

In contrast, R-134a was introduced and became the replacement for R-12, primarily used after 1994 in response to these environmental regulations. R-22, while a refrigerant used in some HVAC applications, was not commonly used in MVAC systems during that period. R-1234yf is a newer refrigerant that began to be adopted later, as it has a much lower global warming potential compared to its predecessors, and it wasn't available in MVAC systems before 1994. Thus, R-12 is the correct answer, as it was the refrigerant that defined that time in the industry.