A. By compromise.
In a democracy, citizens elect the ones who will rule (president, governors, etc.), and once they are in power, the party with the majority of electoral victories sets the agenda. However, there are times when the elected officials might differ in their opinions and there is no clear majority on an issue. When this happens, elected officials trade votes and make concessions in order to agree on legislative action, that is to say, they settle their difference by political compromises.
For example, let's suppose that a Democratic President proposes a healthcare reform to order employers to provide health insurance to all employees, but the Republican Senators disagree and instead proposes a reform that would order employers to provide health insurance only for people that have more than 10 years working in the company. In order to settle the dispute, both parties would have to make a compromise.