Question

A Democrat can vote in a Republican primary in states that

hold open primaries.
hold closed primaries.
have indirect primaries.
have party caucuses.

Answer 1

hold open primaries.

Open primaries allow voters to choose which party's primary to vote in regardless of their party registration. Closed primaries restrict participation to registered party members; indirect primaries and party caucuses are controlled by the parties and generally limit participation to party supporters or delegates.