Which system allows individuals to choose any party primary to vote in?

The correct answer is that an open primary allows individuals to choose any party's primary to vote in. This system is designed to increase voter participation by allowing registered voters to select the primary election of their choice, regardless of their own party affiliation. For instance, a registered Democrat can vote in the Republican primary and vice versa. This flexibility can encourage broader engagement in the electoral process and can lead to more diverse outcomes, as it allows voters to participate in the nomination of candidates across party lines.

In contrast, a closed primary restricts the voting to individuals registered with that specific party, preventing unaffiliated or opposing party members from participating in the primary election. The caucus system involves local gatherings where party members discuss and choose candidates, which is quite different from the primary voting process. A general election is the final election where voters cast their ballots for candidates of the parties at the state and federal level, but it does not pertain to the primary voting process. Thus, the open primary's unique characteristic of allowing voters to select from any party stands out as the key feature in this context.