According to API RP 571 Section 5.3.2.3 (Spheroidization):
"Spheroidization is the transformation of the microstructure of carbon and low alloy steels when exposed to elevated temperatures for long durations. The rate of spheroidization depends on temperature, prior microstructure, and exposure time... The microstructure becomes less effective at carrying loads, and strength is reduced." Key influencing factors for the rate of spheroidization are:
* Temperature (higher accelerates the process),
* Microstructure (initial phase distribution and morphology).
Pressure and hydrogen partial pressure are not relevant for this transformation, nor is stress a primary driver.
Therefore, Option D (temperature and microstructure) is correct.