Where does "75%" come from? If I were trying to estimate what proportion of the population can read, or what proportion of the population is too busy to answer a questionnaire, then I wouldn't assume that the 75% who responded are representative of the whole population. Those are extreme examples but there are lots of things that might be correlated with people's willingness to respond.
Perhaps choice C was: Start again with a new random sample of students, but this time offer FREE FOOD to everyone who responds.
Nobody operating a survey of this type assumes they'll get 100% response rate. (Although it is very common to offer something like a gift certificate to N randomly chosen people who answer the survey.)
Perhaps choice C was: Start again with a new random sample of students, but this time offer FREE FOOD to everyone who responds.