Which is not one of the three most common forms of polls?

a. mail questionnaires

b. in-person interviews

c. door-to-door visits

d. telephone interviews

I think the answer would be (b) in-person interviews.  Usually the polls are conducted through mail questionnaires, especially in the age of internet, and telephone interviews.  Now, we only have in-person interview and door-to-door visits.  Door-to-door is the third most common since this is use by the people in government especially on election, and when conducting a consensus.  So, b would not be in three most common forms of polls.