Answer:
c. student freshmen email addresses
Explanation:
Sampling frame can be most simply defined as a pool from which the sample is taken. So, there can't be any sample taken from outside of this frame.
Some may notice that it has similar definition as population, however, population is more general, it just definea the limits of sampling frame, whereas sampling frame is set of all data or people that can be sanpled.
In this particular case, the population would be all the freshmen at Boozeman State University; they are tho ones who can be sampled.
Sampling frame is more specific; it is the list of every member of the given population. In this case, students are listed by their email addresses.
So, the professor will choose his sample, from the sampling frame which consists of all freshmen email addresses.