What describes a primary where anyone can vote to choose a presidential candidate?

An open primary is a type of primary election where any registered voter can participate in the selection of a party's candidate for the presidential election, regardless of party affiliation. This means that voters do not have to be members of a particular political party to vote in that party's primary. The accessibility of open primaries allows for a broader range of voter participation and can potentially lead to more moderate candidates being nominated, as they must appeal to a wider audience that includes independents and members of other parties.

In contrast, a closed primary restricts participation to registered party members only, which means that only those who have declared their party affiliation can vote in that party’s primary. This system is designed to ensure that only dedicated party members have a say in choosing their party's candidate. A caucus is a different approach where party members gather to discuss and vote on candidates rather than casting secret ballots. A run-off primary occurs when no candidate receives a majority, prompting a second election among the top candidates.

Thus, the defining characteristic of an open primary is its inclusivity, making it the correct answer for the described primary where anyone can vote to choose a presidential candidate.