We are goinf to analise each case properly
1) As the radio show is asking it could be that their listners do not attend this demand. Then the error would be nonresponse.
2) As the students do the survey based of its readers it depends only of this part of the poblation. Then the error would be voluntary.
3) As the gym ask only to the first 20 members it makes the error would be convenience
So, the correct answer is B.