All of the following are modern polling methods except:
a. pollster interviews
b. mail questionnaires
c. in-person interviews
d. telephone interviews
I can't find this answer anywhere in my textbook. Help!
Mail in are self-selecting. Wasn't that one of the errors in the Truman-Dewey race in 1948?
This explains how samples are selected for polling. Yes, mail-in (and phone-in) samples are self-selecting and not representative of the larger population.
https://www.aapor.org/Education-Resources/Election-Polling-Resources/Sampling-Methods-for-Political-Polling.aspx