The correct answer is letter D
Stanley Milgram (1933 - 1984) was an American psychologist graduated from Yale University who conducted the experience of the small worlds (the source of the concept of the six degrees of separation) and the Milgram Experience on obedience to authority.
This experiment aimed to study individual reactions to concrete indications from others. Obedience was measured through overt actions and implied behavior that was a source of suffering for others.