Answer: The correct answer is d. Karl Marx.
Explanation: The conflict theory was proposed by Karl Marx.
The theory proposes that any given society can be the perfect playground for a conflict to arise, and certain forces will develop such as oppresor groups and minority groups that will in turn be opressed.
In turn, a conflict of classes arises, and institutions are used to mantain the inequality between ruler classes and the opressed.
In conclusion, the author commonly cited by conflict theorist to form their arguments surrounding social stratification is d. Karl Marx.