There is often attrition when doing surveys. If the correct answer is always A then that implies there can be no attrition for completing the survey. Since the

There is often attrition when doing surveys. If the correct answer is always A then that implies there can be no attrition for completing the survey. Since the list of topics asked don't seem to correlate with non-responses (such as doing a survey about sexual harassment where non-responses may be very important), the correct answer would be likely D, to discard the subpopulation.