To achieve both anonymity and flexibility in questioning, the researcher should use an Internet survey.
Internet surveys allow respondents to maintain anonymity since they can complete the survey online without revealing their identity. This is particularly relevant for sensitive topics like a sexual survey, where individuals may feel more comfortable providing honest responses if their identity is protected.
Internet surveys also offer flexibility in questioning as they can include a variety of question types such as multiple-choice, open-ended, or Likert scale questions. Respondents can answer the questions at their own pace and in their preferred environment, providing convenience and flexibility for participants.
Therefore, the most suitable option for the researcher to conduct a sex survey, considering the need for both anonymity and flexibility, would be the D. Internet survey.