Which of the following is the best way to conduct a study? Mail questionnaire, telephone interview, and personal interview.
This depends on what sort of a study you are trying to conduct. A mail questionnaire is certainly the best for a study in which you need to ask a lot of people questions that are easily answered. However, if you need fewer respondents and have more in-depth questions, a personal interview might be best.
A questionnaire has the advantage of being a cheap and easy way to contact a lot of people. It is much easier than trying to contact a large number of respondents, even by phone.
Personal interviews and phone interviews are both good because they can allow you to ask in-depth questions and to interact with the respondent. Personal interviews give you a better rapport with the respondent and can therefore help get them to be more open with you. However, phone interviews are easier to arrange and cheaper/easier to conduct since you need not travel to get to the interview site.
With these advantages in mind, you would have to look at what sort of a study you are doing to determine which method is best for you.