The correct answer is A. They try to get their members elected whom they expect to vote along the party platform on issues.
Political Parties have their political platform - Democrats defend social and economic equality, the welfare state, support of labor unions, social programs and others. Republicans defend free market capitalism, strong national defense, deregulation, restriction on labor unions and others - and the way they can influence which platform is transformed into public polity is electing members that can vote in favor of them or not.
All you have to do is write the answer in your thoughts. But just do all of those in your assignment and check if you have done. If this is not what you are talking about, then it might be that you have to chose what you do most.
