a is the best answer choice
Of the system of check and balances.
The system of checks and balances, present in the United States and in many other democratic nations, is a system whereby all three branches of the government oversees, limit and check each other so no part abuse from its powers and a balance in the government may exist. Through this principle, each branch is able to influence the actions of the other.
For example, Congress has the power to enact legislation, however, in such task, the President also has a part. Once Congress passes a bill, it sends it to the President for signature, and the President can either confirm it or it may use their veto power to refuse to sign it. If he or she vetoes the law, it goes back to the Congress where the chambers can use their override power, under certain conditions, to approve the law.
Likewise, the President can make treaties, but the Senate has the power to either approve them or ratify them, and the President can also appoint the Justices of the Supreme Court, but he or she also needs the approbation of the Senate.
The system of checks and balances is used to keep the government from getting too powerful in one branch.