Answer:
judgment sampling.
Explanation:
Judgementbsampling is also called expert sampling is a type of sampling that is based on the assumptions made by an expert. The feedback from this type of sampling tends to be biased.
This is a subjective method of sampling that relies on the researcher's personal judgement in choosing sample from the population.
In this instance the professor chooses only marketing majors as his sample to get feedback on his new textbook. His personal beliefs is that this sample will represent the student population in the United States.
This is however not objective as all the sample members are marketing majors.