The correct answer is option 4) i.e. inhumane/ignominious Let's look at the meaning of all the given words: differentialOf showing or depending on a differenceregalresembling or fit for a monarchcavernA large chamber in a cavefantabulousExcellentsurreptitiousFollowing old traditions without knowing their logicovertDone openlyinhumanewithout humanityignominiousDeserving public disgrace The meanings prove that the correct words to be filled in the sentence should be: inhumane/ignominiousThe complete sentence is: The Prison guards degraded themselves with their inhumane and ignominious treatment of the prisoners.