Which type of primary is characterized by voting restrictions to registered party members alone?

The correct answer is the closed primary, as this type of primary limits participation to registered members of a specific political party. In a closed primary, only those who have declared their affiliation with a party can cast their votes for that party's candidates. This system is designed to ensure that party members can select candidates who align closely with their party's values and policies, preventing non-members from influencing the outcome.

The rationale behind closed primaries is to maintain the integrity of the party's nomination process and to ensure that only those who are committed to a party's ideology have a say in determining its candidates. This can contribute to a more cohesive party platform but can also lead to a lack of inclusivity and potentially alienate independent voters who might support a party’s candidates.

In contrast, other types of primaries, such as open and blanket primaries, allow broader participation, including independents or members from other parties. A caucus, while also a method for selecting candidates, operates differently, as it involves public discussions and deliberations rather than a secret ballot voting process.