Which of the Following Is Not an Example of a Probability

Which of the following is not an example of a probability sample?

A) Asking listeners to call in and vote in a football radio phone-in poll with either a yes or no response.
B) Selecting every 5th name on a list of 100 participants in alphabetical order in order to randomly sample 20% of the group.
C) A random sample of hospital patients is selected to receive a questionnaire about the helpfulness of volunteers on the children's ward.
D) A random sample of participants by age group (20-24,25-29,30-34,35-39) selected as a ratio of the overall population.