According to ANSI C57.

The correct answer isA. UnderANSI C57.12transformer requirements, theaverage winding temperature rise above ambientis commonly limited to65°Cfor standard transformer designs. This limit is important because transformer insulation life is strongly affected by operating temperature. If the winding temperature rise exceeds the permitted value, insulation aging accelerates, reducing transformer reliability and service life. That is why temperature-rise testing is a key verification item during transformer manufacture and factory acceptance activities.

From a source inspection perspective, the inspector should verify that the transformer has been tested in accordance with the applicable standard, that the measured temperature-rise values are properly recorded, and that the results comply with the specified acceptance criteria. The average winding temperature rise is not the same as hotspot temperature, and it must be evaluated against the standard test method and rating basis. The other options are too high for the standard average winding rise value typically associated with ANSI C57.12. In API-aligned source inspection and quality surveillance oftransformers, confirming compliance of factory test results such as temperature-rise performance is an essential part of final acceptance. Therefore,65°Cis the verified answer.