Are long, detailed statements of the party's beliefs and goals that are written at the national conventions.
a. party platforms
b. legislative bills
c. political missions
d. agendas

The phrase that would best complete the sentence, " are long, detailed statements of the party's beliefs and goals that are written at the national conventions" is party platforms. Thus, the answer is letter A. The party platforms are also what are used by the candidates during the debates, forums, and campaigns.

PARTY PLATFORMS are long, detailed statements of the party's beliefs and goals that are written at the national conventions. A party platform basicaly consist of value/principals that the party held in creating it policies. For example, democratic party tend to value larger government involvement in economic policies, and conservative party tend to values less government involvement