Final answer:
Nonresponse refers to questionnaires that are not returned and can introduce bias into survey results.
Step-by-step explanation:
The correct answer is A) Nonresponse. Nonresponse refers to questionnaires that are not returned. This can happen for various reasons, such as people choosing not to participate in the survey, not receiving the questionnaire, or simply not responding. Nonresponse can introduce bias into survey results, as the nonrespondents may have different characteristics than the respondents and may not be representative of the entire population being studied.