The correct answer is B) Restrictions on who can receive public campaign funds have contributed to the persistence of the two-party system.
The diagram that best explains the cause and effect relationship that has shaped U.S. politics is the one that says "Restrictions on who can receive public campaign funds have contributed to the persistence of the two-party system."
Under the political system of the United States, it is very difficult for a third party to be able to compete with success in the Presidential election. Third parties can afford to get the public financial aid and cannot collect the amount of money it is required to compete against the two big American parties: the Republican Party and the Democratic Party. So it is true that The diagram that best explains the cause and effect relationship that has shaped U.S. politics is the one that says "Restrictions on who can receive public campaign funds have contributed to the persistence of the two-party system."