Which of the following is not used by states to determine which delegates will attend the national convention?

a. primary

b. referendum

c. caucus

d. convention

Among the following options, referendum is not used by states to determine which delegates will attend the national convention. The correct option among all the options that are given in the question is the second option or option "b". Referendum is the process of direct voting in which the total electorate is asked to vote on a certain proposal.