(a)ProfessorsDoctorsMen(b)MenDoctorsProfessors(c)ProfessorsDoctorsMen(d)ProfessorsDoctorsMenWhich diagram best represents the relation between professors, doctors and men?

The three classes overlap only partially, so three mutually intersecting circles fit.

Some professors are also doctors, so those two groups share a region.

Not every professor or doctor is a man, and not every man is either.

So no group lies fully inside another and none stays fully separate.

Three overlapping circles show every shared and unshared part.