In addition to a primary political parties can nominate candidates for general election through a petition or a vote of party leaders caucus or a convention signature drive or public rally referendum or a ballot initiative

The correct answer is b). caucus or a convention.

In a caucus, party members have reunions in their district and commit to support their favorite presidential candidate. This results in having a certain number of delegates. These delegates announce who is their favorite candiate trying to influence people’s vote.

After the primaries and caucuses are finished in the states, the party organizes a national convention to vote and then announce his candidate for president.  

use wikipedia, i think they have this.

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