[mindslight]

That still doesn't fix the problem with the question. Without knowing the goal of the survey (what it is trying to measure), you can't determine the (in)significance of the non-responses.

Each answer has a failing:

A - student didn't have much to say, answers tainted by student being annoyed and clicking through the survey to stop the spam

B - surveyor didn't like the answer they got, so they drew again. that's straightforward biased sampling

D - might not make for enough data, as you said

E - student body isn't stateless, and may have had time to talk about the survey. "Hey friends, let me know if any of you get asked to do that survey because I want to give them a piece of my mind"

Choosing between them is only possible by making some simplifying assumptions, which requires knowing what you're trying to find out.