Final answer:
A runoff election is most likely to occur in a First Past the Post or majority election system. If no candidate wins a majority in the first round, a runoff between the top candidates is held. This type of election is not typically associated with proportional representation, single transferable vote, or ranked choice voting systems.
Step-by-step explanation:
A runoff election is most likely to occur in an electoral system that requires a candidate to receive a majority of votes to win. This system is known as a majority election. When no candidate secures more than 50 percent of the votes in the initial round, a runoff between the top candidates is conducted. Thus, the correct answer to the question is A) First Past the Post, as it's typically associated with majority rules and might lead to a runoff if no candidate achieves a majority in the first round of voting.
Looking at American politics as an example, states conduct primary elections within the two major political parties in a similar majority and runoff approach. While the first-past-the-post system is prevalent in the U.S., when elections require a true majority, runoff elections may occur, as seen in certain local and non-partisan elections. This is distinct from systems like proportional representation, single transferable vote, and ranked choice voting, which allocate seats or elect candidates based on different criteria that often don't result in runoffs.