Which category of compressor "traps" a volume of refrigerant vapor prior to compression?

The correct choice is associated with positive displacement compressors due to their operational characteristics. Positive displacement compressors work by trapping a specific volume of refrigerant vapor within a defined space before it is compressed. This is achieved through a mechanical action such as a piston moving in a cylinder, vanes moving in a rotor, or other mechanisms that create a suction effect. As the volume within the compartment decreases, the refrigerant is compressed, leading to an increase in pressure.

Scroll, centrifugal, and rotary compressors operate differently. Scroll compressors utilize two interleaved spirals to compress the refrigerant and don't "trap" vapor in the same manner, as they continuously move the refrigerant through the spirals. Centrifugal compressors, on the other hand, rely on the kinetic energy generated by rotating impellers to achieve compression, which involves continuous fluid flow rather than trapping. Rotary compressors use a rotary mechanism but also focus on continuous flow rather than the trapping of vapor. Thus, positive displacement compressors are uniquely characterized by their ability to trap refrigerant vapor prior to compression, making this option the most appropriate answer.