What is the defining feature of an open primary?

An open primary is characterized by the ability of voters to participate in the selection process of candidates regardless of their party affiliation. This means that individuals who are registered with one political party can choose to vote in the primary elections of another party. This approach encourages broader participation and can lead to a more inclusive electoral process, allowing individuals to have a say in candidates from multiple parties, rather than being restricted solely to their own.

The other options reflect features that are associated with different types of primaries. For instance, the concept of voting only for candidates from one's own party pertains to closed primaries, where only registered members of a political party can vote in that party's primary. Similarly, limiting voting to registered party members is also indicative of a closed primary system. Finally, the occurrence of primaries only in presidential election years does not define the primary system itself, as primaries can happen during midterm elections as well.