Question

what is the purpose of each political party holding a national convention every 4 years?
to announce their political parties presidential candidate
to declare who is on the primary election ballot for each party
to decide who are the members of the presidents cabinet

Explanation:

In the context of U.S. political parties, the main purpose of a National Convention (held every 4 years) is to officially announce the party's presidential candidate. Primary elections are for selecting candidates to move forward, not what the convention declares for the primary ballot. The President's cabinet is chosen by the elected President, not at the party convention.

Answer:

A. To announce their political parties Presidential Candidate