How are the republican and democratic presidential candidates chosen?

Republican and Democratic parties decide their presidential candidates by carrying on primary presidential elections and caucuses.

Explanation:

To decide their candidates, the parties in the United States of America run democratic processes in which the public has power in the decision of choosing them. However, they also run intern processes to decide them. Both are carried on in only certain states, as well as they are run January and February to finish the whole process in June to have enough time for the candidate to prepare for the general election in November. The U.S. constitutions have never established the process, the parties have managed it and they have established their own rules.

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c is the correct answer for this question.