Which of the following best describes a closed primary?

A closed primary is best described by the characteristic that only registered party members can vote in it. This system is designed to ensure that only those who are officially affiliated with a political party can participate in selecting that party's candidates for the general election. By restricting participation to registered party members, closed primaries aim to prevent members of other parties from influencing the selection of candidates in a way that might detract from the party's values or interests.

In contrast, open primaries allow any registered voter to participate in either party's primary, regardless of their party affiliation, which introduces a different dynamic to the electoral process. The option mentioning online conduct refers to a method of voting rather than the nature of participant eligibility, while the choice about eligibility pertains more broadly to the general voting population and does not specifically address party membership requirements, which is the hallmark of closed primaries.