The correct answer is B) cooperation between Congress and the executive branch.
A rise in party unity is indicative of cooperation between Congress and the executive branch.
Under the division of powers in the United States Federal government, the system of checks and balances allows the equilibrium between the three branches of the federal government. None is the above or below the other. That is why it is so important for the country that the Executive branch (the President) and the Legislative branch (US Congress) have a good relationship and communication. This way, open lines serve to maintain a productive relationship in the passing of proper laws and work together in the best interests of the nation.