FWIW, ChatGPT answered this in one go:

"It is best for the university to use the 90 questionnaires that were submitted as the final sample. Since the university selected a random sample of 120 students and only 90 responded, those 90 responses provide a representative sample of the opinions of this year’s freshman class. Starting over with a new sample or adding 30 more students to the sample could introduce bias into the results. Additionally, sending another email to the 30 students who did not respond may not be effective in increasing the response rate."