The answer is D. The national party.
In a political party organization, the structure typically moves from local to national levels. The national party is generally considered to be at the top of this hierarchy, as it oversees the party's operations across the whole country, sets policies, and coordinates campaign strategies. Local organizations and state parties function under this national umbrella, managing grassroots efforts and representing the party at regional and local levels. Volunteers play a crucial role within these local and state structures, but they do not comprise the organizational hierarchy itself. Thus, the national party represents the highest level of organization within that context.