Final answer:
A runoff election is most likely to occur in a majority system, where a subsequent election is held if no candidate achieves more than 50% of the votes in the initial election. Therefore, the correct option is 2) a majority system
Step-by-step explanation:
A runoff election is most likely to occur in a majority system. In this type of electoral system, if no candidate wins more than 50% of the vote, a subsequent runoff is held between the top two candidates. This contrasts with a plurality system, where the candidate with the most votes wins even if they do not secure a majority.
Proportional representation, on the other hand, allocates seats based on the percentage of votes each party receives, rather than requiring a majority or plurality for individual candidates. Lastly, closed primary systems are internal party processes, not general elections, so they do not typically feature runoffs in the same way.