What Is the Main Function of the Presidential Primary

What is the main function of the presidential primary?

A) to give the candidates an opportunity to voice their intentions, platform, and agenda
B) to give the national party an idea of whom the voters favor and would be likely to vote for
C) to weed out the presidential candidates who are not viable as the party's nominee
D) to allow voters some say as to who they want to see as their party's presidential nominee