Which of the Following Examples Would Most Likely Be Subject

Which of the following examples would most likely be subject to selection bias?

A) 1,000 registered voters in Nashville are called and asked who they are planning to vote for in the upcoming Nashville mayoral election.
B) 500 University of Wisconsin students are polled on their opinion about a possible tuition hike at their school.
C) 750 National Rifle Association members are questioned to obtain the public's opinion about pending federal legislation regarding stricter gun control laws.
D) 60 members of a 100-member neighborhood homeowners' association vote to approve funding for private security patrols in their neighborhood.