How Should a Researcher Select Subjects for a Study in Order

Multiple Choice

How should a researcher select subjects for a study in order to generate results that are generalizable

A)  Subjects should all be chosen from the same geographical area and socio-economic class. 
B)  Subjects should be allowed to choose which group they would like to be in. 
C)  Subjects should be from WEIRD societies. 
D)  Subjects should be carefully chosen so that they are a representative sample of the population.