Answer: Quota sampling
Step-by-step explanation:
Given : Dr. Smith is instructing his graduate students to put together a sample for an upcoming research study of college students.
The graduate students were asked to stand outside of the student union to solicit participants, finding 50 freshmen, 50 sophomore, 50 juniors, and 50 seniors.
Here, the characteristic individuals are chosen out of 4 specific subgroups.
Thus, the sampling method is used here is Quota sampling.