__________ law of thermodynamics ascertains the direction of a particular spontaneous process.

Answer: Option C

Second law of thermodynamics is the only law that implies an directional constraint on any naturally occurring process it basically say entropy of an isolated system(universe) always increases. $$\Delta S_{universe}⩾0$$
So, for an system undergoing some process during which its entropy is increasing than we can say the system is undergoing spontaneous change but here the system should undergoes constant internal energy change and constant volume change if the system is allowed to do only expansion work.