Which is the best example of the seniority rule?

Which is the best example of the seniority rule?

a. A chairman decides the priority of each bill being considered by a standing committee.
b. A four-term senator is appointed to an important job over a first-term senator.
c. A party caucus decides who should be the Speaker of the House before Congress organizes. d. A party whip encourages caucus members to vote for a bill supported by the party leader.