Answer: B, Randomly select 6 graduate students and 24 undergrad students for the survey.
Step-by-step explanation: This guarantees that each type is proportionately represented in the survey.
Not A because it risks too many or too few graduate students. It is not C because again, it risks too many or too few graduate students. It is not D because each section has about 50 students and the professor only wants 30 in his sample.