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Superheat is the amount by which the refrigerant vapor is heated above its saturation temperature at the same low-side (evaporator) pressure. In practice, you measure the suction-line temperature and subtract the saturated vapor temperature corresponding to the suction pressure; the difference is the superheat.

This definition is why the correct statement matches: it uses suction-line temperature and the saturated vapor temperature at the low-side pressure. It confirms that the vapor leaving the evaporator has been warmed beyond its boiling point at that pressure, which helps prevent liquid from reaching the compressor and provides a useful diagnostic of evaporator load and system charge.

The other ideas don’t describe superheat. Subcooling relates to the liquid line on the high side and compares liquid temperature to its saturated liquid temperature. The compressor’s temperature rise is about how hot the gas gets inside the compressor, not the vapor’s extra heating above saturation. Ambient air temperature has no direct role in defining superheat.