The correct answer is C) temperature readings at normal loads to detect abnormal temperature rises. During routine AC motor maintenance, checking the temperature readings at normal loads is crucial to detect any abnormal temperature rises. This is an important preventative maintenance step to identify potential issues with the motor before they become more serious problems. Abnormal temperature rises can indicate issues like winding insulation breakdown, bearing problems, or other internal faults that require attention. The other options, while potentially useful in some cases, are not considered part of the standard routine maintenance procedures for an AC motor. Inspecting the motor's internals (A) and verifying the RPM (B) are more specialized tasks, while a rotor balance check (D) is generally only necessary if vibration issues are suspected.