What is the key difference between closed and open primaries?

The key difference between closed and open primaries lies in who is allowed to participate in the voting process. In a closed primary, only registered members of a political party can vote to select their party's candidate for the general election. This means that if you are a registered Democrat, you can only vote in the Democratic primary, and the same applies to registered Republicans and other parties. This system is designed to ensure that only those who have a vested interest in the party's platform and candidates influence the outcome of their party's selection.

The other options reflect misunderstandings of how primaries work. Open primaries do not require participants to be registered members of a party to vote in that party's primary, which directly contradicts the nature of closed primaries. Moreover, open primaries can occur in various types of elections beyond just presidential ones, making the assertion that they are only held during presidential elections inaccurate. By clarifying what closed primaries entail, it becomes clear why it is the correct answer.